Despite Kashmir being a disputed territory as per the UN, India still went ahead with fully integrating Kashmir by revoking Article 370. What has prevented Pakistan from fully integrating AJK/PoK?

 Very good question. What preventing Pakistan from fully integrating AJK/POK >

It's not that Pakistan doesn’t want to, but it has been smartly given a Checkmate.

Pakistan is in a catch-22 situation. Internally there would be pressure from the mainland Pakistani people on their Government to attach POK/AJK as a counter-measure of Indian move as well to prevent any future Indian attempt to recapture POK.

However, the moment they make any such move they will lose the support of the POK people. The POK people may revolt against the Pak government. A hostile POK will affect its strength adversely along LOC. India Government would get the upper hand in improving the situation in J&K as well as making advancements in POK. Pakistan Government will lose the mask/moral right to fight for freedom of J&K. Their international stand will get diluted. And if they do not take a call to attach POK, there will always be an opportunity open for India.

Imran Khan's Government was in a state of total confusion and frustration. They were not getting any support from the international community, not even from the OIC countries. OIC countries are rather standing with India. FATF Asia-Pacific Group had already put Pakistan in an 'enhanced blacklist, financially they were not in a position to fight any sustained war with India. Their military might is no match to that of India. Defence Minister Rajnath Singh already said that the future of India’s ‘No First Use’ (NFU) policy on nuclear weapons depended on “circumstances”. Mr. Singh’s statement has raised apprehensions about the likely revision of India’s NFU policy and nuclear doctrine.

Pakistan knows that whatever counteraction they have taken ( after India removed Article 370 ) would hurt them more than India and at some point in time they would be forced to call off those actions. Which would make it lose its face badly to its own people as well as to the world community.

Further in such a condition if they decide to wage a war against India as they are threatening they will not get any support from any corner and it will give India a legitimate opportunity/right to go all out and recapture POK. Possibly that’s the reason they are talking about nuclear war so often, so that the world community intervenes and stops India from making any further moves.

Pakistan understands that Article 370 is gone forever, it can not come back. Now POK is at risk. If they are to have any discussion in the future it would be on POK not on J&K.

The best thing they can expect now is for the International community to put pressure on India and extract some kind of statement from India that it would not make any unilateral move in POK. But to get such a statement from India International community has to give an absolute guarantee to India that Pakistan would not interfere in J&K and would stop all kinds of proxy wars against India.

And till such time Pakistan gives such a declaration and stops all terrorist activities in J&K, the International community can not put any pressure on India. Overall no gain for Pakistan.

For the first time, India Government has put the ball in Pakistan's court and pushed it on its back foot. A brilliant tactical move by the Modi Government.