User blog comment:A.J AL/Season six plot hole./@comment-5619706-20121215033415/@comment-3045936-20121216180258

It could be that unless they are pulled out and revived, souls normally never leave Heaven. Like, also Castiel was still more good at the time, so to use the souls of Heaven would be a bad thing. Using the souls from Hell or Purgatory in Castiel's mind wouldn't be harming God's laws (if there are any) as those places are like jails. So in Castiel's mind it wouldn't be wrong to use those souls as they as a majority generalization not good. Another reason is, logically the writers may never had thought of it.