Probably because Uruguay didn't do it either and apparently [without consequences](https://www.cbc.ca/news/politics/canada-cannabis-pot-treaties-1.4566987):

> While Canada appears set to violate these international agreements, the [International Narcotics Control] board has limited powers when it comes to forcing signatories to comply.

> Gélinas-Faucher said that the agency can ask countries which signed these agreements to discontinue their drug trade with Canada, since the agreements govern trade in legal drugs as well. And another country could sue Canada before the International Court of Justice.

>Those scenarios, however, are considered unlikely. Uruguay, which also legalized cannabis, has not been subjected to similar reprisals.


And here's what happened in [that case](https://www.brookings.edu/wp-content/uploads/2018/03/gs_032118_uruguaye28099s-cannabis-law_final.pdf), basically Uruguay argued that human rights trump drug enforcement:

> Uruguay has forged ahead with cannabis regulation despite repeated criticisms from the International Narcotics
Control Board (INCB), as in the Board’s report for 2016, which states:  
>>The Board notes the continued implementation by the Government of Uruguay of measures aimed at creating a
regulated market for the non-medical use of cannabis. While this policy has not yet been fully implemented, the
Board wishes to reiterate its position that such legislation is contrary to the provisions of the international drug
control conventions, particularly to the measures set out in article 4, paragraph (c), of the 1961 Convention as
amended, according to which States parties are obliged to ‘limit exclusively to medical and scientific purposes
the production, manufacture, export, import, distribution of, trade in, use and possession of drugs.’

>In opting for regulation notwithstanding such criticisms, **Uruguay has argued that its policy is *fully in line* with the
original objectives that the UN drug control treaties emphasized but have subsequently failed to achieve: namely, the
protection of the health and welfare of humankind.** While there can be little doubt that, as the INCB has pointed out,
Uruguay is contravening its obligations under the 1961 Single Convention to limit cannabis exclusively to medical
and scientific purposes, Uruguay has sought to side step the question of drug treaty non-compliance by placing
its new law in the context of the country’s adherence to its more foundational obligations under international law.

> Uruguayan authorities have specifically argued that the creation of a regulated market for adult use of cannabis
is driven by health and security imperatives and is therefore an issue of human rights. As such, officials point to
wider UN human rights obligations that need to be respected, specifically appealing to the precedence of human
rights principles over drug control obligations within the UN system as a whole. In the event of a conflict between
human rights obligations and drug control requirements,
they argue, Uruguay is bound to give priority to its human
rights obligations.

> The argument for the priority of human rights obligations
in matters of drug control is not a new one for Uruguay. In
2008, Uruguay sponsored a resolution at the Commission
on Narcotic Drugs (the UN’s central policy making body on
the issue) to ensure the promotion of human rights in the
implementation of the international drug control treaties.
**Uruguay’s argument that human rights protections take
precedence over drug control requirements finds support
in the 2010 report to the UN General Assembly by the UN
Special Rapporteur on the Right to Health, which signaled: “When the goals and approaches of the international
drug control regime and international human rights regime conflict, it is clear that human rights obligations should
prevail.”**

> In 2015, Uruguay co-sponsored a resolution calling upon the UN High Commissioner for Human Rights (OHCHR)
to prepare a report ‘on the impact of the world drug problem on the enjoyment of human rights.’16 In its contribution
to OHCHR preparations, Uruguay laid out its stance regarding the primacy of human rights:

>> We reaffirm the importance of ensuring the human rights system, underscoring that human rights are universal,
intrinsic, interdependent and inalienable, and that is the obligation of States to guarantee their priority over
other international agreements, emphasizing the international drug control conventions.

> Uruguay’s ability to move forward with a policy clearly beyond the bounds of the UN drug treaties owes to a combination
of factors. First, Uruguayan authorities foresaw the international criticism their move would likely trigger,
and fashioned an argument based on human rights obligations that was consistent with the country’s international
reputation, and that was coherent with the country’s rationale for revising its cannabis law in the first place.
Second, as a practical matter, the UN drug control treaty bodies, including the INCB, do not have the kind of
enforcement authority or practical political power that would be necessary to prevent Uruguay from moving ahead
with implementation of its new law. Countries such as the United States have historically wielded their political
influence and power to encourage full implementation of the drug treaties. **However, with Uruguay’s law entering its
*fifth year* since passage, there has not been a concerted U.S. government effort to punish Uruguay bilaterally or in
an international arena, suggesting that Uruguay’s reforms will not be stymied because of international pressures.**
In this regard, Uruguay has taken advantage of felicitous timing, with its law’s passage having come in the midst of a
major shift toward cannabis regulation within the United States. After the November 2012 ballot initiatives to legalize
cannabis in the states of Colorado and Washington, U.S. President Barack Obama’s administration adopted a policy
of conditional accommodation of state-level cannabis legalization, contained in Justice Department enforcement
guidance known as the “Cole Memo.”18 This accommodation provided Uruguay a political cushion internationally,
just as the Uruguayan parliament was preparing to approve the country’s cannabis reform.    
In the wake of the Colorado and Washington ballot initiatives, the U.S. federal government was suddenly in an
awkward spot. The United States was the key architect and for decades the chief enforcer of the UN drug treaties,
including vigorous enforcement of the global prohibition on non-medical uses of cannabis. To oppose Uruguay’s
new law or even pressure Uruguay to revise or annul it—as it is easy to imagine previous administrations attempting
to do so—would open the United States to charges of hypocrisy.

> Indeed, regarding non-medical cannabis, **the INCB has also repeatedly noted that the United States is “not in conformity”
with the drug treaties,** and has underscored that the “strict prohibition of non-medical use set out in the
1961 Convention” applies fully to countries with federal structures of government. In other words, if “sub-national
Governments have taken measures towards legalizing and regulating the non-medical use of cannabis, despite
federal law to the contrary”—as is quite evidently the situation today in the United States—then such developments
are “in violation of the international drug control legal framework.” In this new context, the United States has kept
its criticisms of Uruguay’s cannabis law soft and perfunctory.

> Under President Donald Trump, U.S. Attorney General Jeff Sessions has made clear his animus toward legal cannabis.
In January 2018, Sessions rescinded the Cole Memo, heightening concerns over how federal enforcement powers
will be wielded. But Sessions’ bid to turn back the clock on cannabis legalization is considered unlikely to succeed;
cannabis reforming states are not expected to be reverse course, even if the Trump administration expends significant
political capital in an effort to compel them. This leaves the U.S. federal government in the same awkward situation
that began in November 2012 when the voters of Colorado and Washington State approved their ballot initiatives:
unable to undo the states’ reforms, and therefore out of compliance with the drug treaties it has long championed. For the foreseeable future, the United States is unlikely to be in a position to oppose efforts to legalize and regulate
cannabis, such as that now underway in Uruguay.

It looks like Canada is taking [a somewhat similar line](https://sencanada.ca/content/sen/committee/421/AEFA/Reports/AEFA_BILLC-45_Report_Final_e.pdf) (to Uruguay's)...

> Witnesses appearing before your Committee all agreed that if Bill C-45 is passed,
Canada would be in violation of the 1961 Single Convention, the 1971 Convention on
Psychotropic Substances, and the 1988 Anti-Trafficking Convention.    
> Your Committee heard from the Minister of Foreign Affairs at the end of its hearings. In
her testimony, the Minister “recognize[d] that [the] proposed approach of legalizing,
[regulating], and strictly restricting cannabis will result in Canada contravening certain
obligations […] under the three UN drug conventions: the Single Convention on
Narcotics Drugs from 1961, the 1971 Convention on Psychotropic Substances and the
1988 United Nation Convention Against Illicit Traffic in Narcotic Drugs and Psychotropic
Substances.”
Even as Bill C-45 will violate certain international obligations related to cannabis, **the
Minister of Foreign Affairs noted that Canada’s policy regarding international drug
control remains *consistent with the overarching goal of the three conventions*, namely
to protect the health and welfare of society.** Moreover, while **GAC officials in earlier
testimony characterized the contravention of the international drug conventions as
*technical***, the Minister of Foreign Affairs stated before your Committee that “the issue
of the conventions is an important one, and [the government] need[s] to be clear about
it.” [...]

>  She informed your Committee that government officials
have been discussing Canada’s intention to legalize recreational cannabis with G7
countries as well as a dozen others. In particular, she noted that Canada was
re-elected to the Commission on Narcotic Drugs (CND) in a contested election in
March 2017, after having announced its intention to legalize recreational cannabis.

> In answering questions put to her about how Canada intends to reconcile its views on
the importance of a rules-based international order with Bill C-45’s violation of
international treaties, **the Minister pointed out the need for Canada to be *open about
being in contravention***, to preserve the health and safety of Canadians, as well as to
work with international partners. 

And yes, INCB also protested against Canada in similar terms as against Uruguay; I'll spare you the quote. Their position will probably have no effect.