User blog comment:TheRealD/Why won't Doflamingo uses his parasite ability on everyone in Dressrosa?/@comment-24.20.234.177-20141228185130

I think he does the game for fun because we all know he is incredibly sadistic so under the assumption that the his crew can take down the Straw Hats and he can take down Luffy and Law he doesn' really need to in his mind.

Another thing to mention is he thinks that Luffy is "just another pirate" so he may be underestimationg him. Also he is completely un aware of Sabo's power.