A) There's plenty of plausible speculation that AQ already has nukes, much on that topic has been linked and posted in this thread.
B) Surely AQ doesn't believe it could ever get successful access to Pakistan's strategic nukes. India, Israel, the US, and probably others would simply not let that happen under any conceivable circumstances.
C) AQ does need a friendly operational base. Pakistan already serves in that fashion but AQ is probably cowed by the number and intensity of foreign intelligence operatives there. They'd like a more sympathetic government to throttle the intelligence ops against them.