User blog comment:TheRealD/Why won't Doflamingo uses his parasite ability on everyone in Dressrosa?/@comment-1595565-20150108061422/@comment-10543621-20150108065220

But what's the limit? If the limit is simply based on the amount of 'strings' he's currently using such as maintaining the birdcage and controlling random citizens, wouldn't it be more beneficial for him to release a few random citizens so that he could actually use the ability on deadlier foes instead?

I think a limit on the amount of string used at one go is a teneous reason at best. I'm hoping for a more concrete sort of limitation to his abilities being explained. Like maybe he can only grab people with proper solid bodies in order to control them with parasite, and being able to stretch makes controlling complicated. So it's easier for him to try and cut Luffy instead. Not controlling Law can be somewhat explained due to his Room ability as well.