its because of Pakistani perceived weakness that India had the boldness to make this move. Pakistan really needs to re-assess its strategy, and ask itself what it wants to sacrifice for Kashmir.
Pakistan treated their Bengali citizens like second class citizens, and india fully took advantage of that and ended eastern Pakistan.. whos fault was that?
India is a massive state with a multi-ethnic make up. There have even been open conflict between indian sub-groups (golden temple massacre of Sikhs as 1 example )
has Pakistan ever taken advantage of this? india has hundreds of millions of muslims has Pakistan ever taken advantage of this? the biggest way you could hurt india is to play the same ethnic card they played against you in Bangladesh.
do everything you can to spark ethnic unrest between the various groups. that is the biggest weakness of india. that's how you defeat india. Pakistan either hasn't gone this route, or has failed miserably trying.
and lastly it comes down to Pakistani sacrifice and willingness to go to war if necessary. If you want to keep up the conflict with india. then you need credibility. You need to increase your aggression. If an indian army shells you, You take a radical step to escalate (airstrike on an indian position). you take the lead and not just react to indian actions, and be ready for war if it comes.
when you act gun shy, and decrease your defense spending, and have a mild prime minister whos only interested in diplomacy then the Indians sense weakness, and calculate that they can get away with a move like this.
Pakistan needed to make this their red-line. and threatened open nuclear war if India made sucha move. But it never did.
Pakistan is simply losing the geopolitical game to india.