well.. please don't get me wrong here..
I feel it has to do with the basic attitude of people.. Pakistanis in general are aggressive, instinctive and spontaneous.. and indians in general are patient, methodical and calculative...
it refelects in their game as well. A fast bowler should have all the traits, all the fast bowling greats of Pak had those attributes (there might be a couple of exception).
Whereas to be successful in batting one need to be methodical and calculative, even sehwag has a method to his approach, he is very calculative and methodical. Pak batsman lacked this ability (there are exceptions like hanif mohammad, miandad, yusuf and abbas, but in general pak batsman lack these attributes)
Even the good bowlers that india produced like srinath and kapil, and spinners like dedi, prasanna, kumble and venkat are all methodical, they work slowly, set a batsman up and earn their wicket.
They waited for the pitches to wear out, and use the pitch to aid them.. where as even spinners in pak are not methodical, despite being highly talented they have the attitude of a fast bowler..
It is just my opinion, please don't take any offence.. some might say it has to do with gene and stuff.. I don't buy that argument..