Lol because Oda does not consider Doflamingo a big player and he is a bigger player than any Supernova, including Luffy.
I know Luffy has done outrageous things but that just makes him a nuisance in the eyes of the WG, not someone who can seriously oppose them, like the Yonkou seem to be able to do.
Doflamingo is the biggest player in the underworld, is not afraid of Aokiji, is the king of a country, a partner of Kaidoh, made a logia vice admiral his bitch, and considers Law, a Shichibukai, not yet ready to be one of his subordinates, as he hadn't given Law the heart seat yet, and he is the Shichibukai that the WG has enough faith in to kill other Shichibukai.
Doflamingo is a big player, besides being strong himself, he has many strong subordinates(Diamante, Pica, Trebol all supposedly as strong/stronger than Law), and his position of King of Dressrosa, Shichibukai, Kind of a World Noble, and the Joker of the Underworld gives him a ton of political power with both the WG and the criminals of the world. He is one of the biggest players there are besides the Yonkou.
Luffy and Law are big players as well, just like all the other members of the worst generation, they just aren't the biggest players of the world yet, all of the supernovas from 2 years ago are most likely dangerous enough to fight evenly/defeat a Shichibukai, which means that as a group, they are as strong/stronger then one of the three great powers of the world, not counting their crews, thats why the world panicked when they started forming alliances, imagine how much crazier the Dressrosa Arc would be if Laws crew was there causing havoc as well