Like I said in the previous post, there is a language Continuity on the (roughly) right side of the Indus river.
People from Kashmir speak Pahari language which is understood by people living next to them: Hazara
People from Hazara speak Hindko language which is understood by people living next to them: Pothohari
People of Pothohar speak Pothohari language which is understood by the people living next to them: Punjabis
People from (central) Punjab speak Punjabi language which is understood by the people living next to them: Siraikis
People from Siraiki area speak Siraiki language which is understood by people living next to them: Sindhi.
The same sequence can also run backwards from Sindh to Kashmir.
I think that the biggest geographic barrier making West Pakistan's languages slightly less intelligible is the River Indus, which must have been a mighty barrier in the past. But, even these languages are derived from Persian languages, the same languages from which Urdu gets most of its vocabulary.
My point is that, Urdu owes most of it's grammar and vocabulary from these languages and can be considered natural inheritor of these languages and this particular area. Hence, Urdu is the most appropriate language to be made as a National language.