Perhaps because only Law and Sanji have fought Doflamingo? That's why he's done more damage to him than anyone else. Mingo didn't ever receive damage until chapter 722 (I believe). The second time? Not like he just cut off Laws arm, rendering him helpless or anything (Doflamingo only used three techniques in the second round).
I don't think Luffy's gone all out. An all out Luffy now is a Luffy that pulls off a Gomu Gomu no Giganto Thor Gatling. Although, Doflamingo would easily put a holt to it due to the nature of the Ito Ito no Mi.
Trebol can't even fight properly, he's unable to pull himself back together. He's useless other than sneezing on Law (literally). Trebols a non-factor in this. One Haki imbued attack strong enough (given the opportunity) would wipe him out now. And both Law or Luffy can deliver it.
Because of the Ito Ito no Mi's ability to render almost anybody immobile I believe that (even though the M3 and Law etc. would be low diffed) Luffy, Law, Zoro, Sanji aren't much weaker than him. They simply lack the immense Busoshoku Haki to fight the strings or the Kenbunshoku Haki to see them coming from miles away. You'd only need to go one level up in terms of either to defeat Doflamingo, but the nature of his fruit makes him difficult to fight by yourself. It's the nature of his fruit. Don't get me wrong, I think they're close in power in a 1v1, but you just can't fight Doflamingo unless you're substantially stronger than him, otherwise you have to tag team him.