a10khan,
The biggest problem is accomodation. Sahiwal, Jhang, Gujarawala, Sialkot, Bahawalpur, Okara, Kasur, Sheikhupura, Sarghoda and Attock have big populations but all of these venue's lack an international standard hotel or even a similar standard ground. Can you picture a team like England, with its bevy of followers and flocks of journalists agreeing to spend 3 days in a 2/3 star hotel in, say, Sarghoda? Granted, Sheikhpura and Gujrawala are within a couple hours of drive from Lahore but the logistics of organising such trips, with the current focus on security, would be nightmarish.
The only venue's PCB can work with are Multan, Lahore, Fiasalabad, Rawalpindi, Karachi and Peshawar.
That being said, IMO, PCB oughta develop some more grounds, specially for ODI's. How about one in Murree, or up north in Abbotabad? Those places are riddled with top quality hotels and the pitch would likely make an interesting contrast with the rest of Pakistani tracks. The elevation might cause a problem though, but hasn't India got grounds at similar height in Hamachal Pardesh?
Queeta is a great venue as well, though It's proximity to Afghanistan makes it a no-go area for most visitors.