Short answer: Cash. Or rather the lack of it. Both PIA dn their owners the Govt are strapped for cash, perenially seeking extensions on their loan repayments. Unlike the other South Asian countries, the All the growth that you talk about has come through cash injections from foreign aid donors, and Foreign investment inflows are at an alltime low.
In this situation, Purchasing aircraft is possibly the LAST in their list of priorities.
The 777's were basically part of the payment that the Pakis received for the support they gave the Americans in their so-called "War on Terror". The 777's are a part payment for dumping the Taliban, a Paki creation.