Why was Imran Khan's plan to naturalize Afghan children born in Pakistan "almost immediately revoked"?

Question

An EU document briefly notes something about

the Prime Minister’s announced plan to naturalise Afghans in Pakistan that was almost immediately revoked

A bit more googling finds a lot more details about what that 2018 plan entailed:

Pakistani Prime Minister Imran Khan, in an unprecedented announcement Sunday, pledged to offer Pakistani citizenship to hundreds of thousands of Afghans born to refugee families his country has been hosting for decades.

"Afghans whose children have been raised and born in Pakistan will be granted citizenship inshallah (God willing) because this is the established practice in countries around the world. You get an American passport if you are born in America," said Khan, who took office last month. [...]

The Pakistani leader explained that since he is also directly overseeing the federal Interior Ministry, which is responsible for granting passports and identification cards, he will instruct his staff to make efforts without further delay to offer Pakistani nationality to the people "who have come from Afghanistan and whose children are raised and born in here."

However, nothing is said there why the plan was quickly dropped, as the (later) EU document claims happened. So who opposed Khan's plan or what made him change his mind?

Answer 1

user366312's answer is basically correct, not only did the opposition parties attack the plan, but also the Baloch nationalist party that was formally part of the ruling coalition did the same, making the passing of any legislation on the matter rather improbable.

Speaking on a point of order, Mr Mengal said when such a statement was issued by a prime minister, it became a state policy. “We need an explanation,” the Baloch nationalist leader said, alleging that such an action would amount to the violation of the agreement reached between his party and the PTI at the time of forging an alliance after the elections.

Somewhat more vaguely stated were concerns that Khan was essentially pandering to his ethnic voters base (Pashtuns), looking to enlarge it.

I'm not entirely convicted that opposition from the BNP was all that consequential, because they only have 4 seats in the lower house nowadays, and probably not that many more during Khan's government either, so it was probably more of a canary/indicator.

I couldn't find what the stated position of the MQM-P was on the matter, but Wikipeida also describes them as a "Muhajir nationalist" party. A year before they were asking for the speedy deportation of all Afghans, so I imagine they weren't too happy with Khan's prposal either. They only have 7 seats, but their defection much later on sealed the fate of Khan's governmet, so I suppose his majority was rather narrow in such ethnically contested matters.