Probably because Uruguay didn't do it either and apparently without consequences:
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, 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.
I'm guessing Canada might be taking a similar line (to Uruguay's), but I haven't found a specific declaration. Clearly however INCB has made a fuss before... but without much consequences.