User blog comment:TheRealD/Why won't Doflamingo uses his parasite ability on everyone in Dressrosa?/@comment-4493594-20141228095624

Doflamingo failed to read the Evil Overlord List. That's why.

It is as it was with every OP villain ever and as it will always be: He believes he has everything under control and underestimates his opponents. Hence, he doesn't feel the need to just kill everyone. Nah, that would be too easy and no fun. We did establish that he's a sadistic a-hole, right?

As for the clones, I think it's a matter of how well you can concentrate/multi-task. I think Doffy needs to control them directly, as it would make sense for his string ability / puppetmaster theme. (Those string clones ain't no Kage Bushin, yo!)

Now, there are just so many things you can do at the same time. And fighting more-or-less-big shots would probably take too much concentration for Doffy to fight more than one or two with his clones at once.