1. It is not 'promised' plebiscite. A promise, by definition is made between two parties where one asks for something and the other, who is making the promise, provides assurance for it.
Pakistan did not ask for plebiscite before UN intervention. Perhaps you are unaware about the events of history, it was Nehru who had gone to UN and brokered a peace deal.
2. As per UN Security Council Resolution 47, Pakistan was required to evacuate all it's tribals and military men pushed into Kashmir first, while India was to maintain a small number of security forces to maintain law and order. That Pakistani withdrawal never happened. Instead Pakistan went on to create 'Azad Kashmir', which is by no means 'Azad'.
Quoting from UNSC Resolution 47 :
3. After the Shimla Agreement, the Kashmir issue had become a bilateral issue and not in the International arena of UN
anymore.
4. Since 2010, UN has removed Jammu & Kashmir from the list of disputed territories.
5. After the Mumbai attacks, it is highly unlikely that any political leader of India will talk to Pakistan about Kashmir and in the process commit a political suicide.
The way I see it, Pakistan should have taken the opportunity when it was presented with one. It should have gone for the plebiscite. Instead Pakistan messed it all up back then, messed it up later in 1965, further mess up in Siachen and Kargil and with brazen support for terrorism. With such a history, one simply does not walk away with unscathed with the cake in hand.