Why does the Legion deserve one more than anyone else?
Because NCR has the major focus during the entire game and has been in a previous game.
House is chained to the ground and can hardly have more to his story outside of Vegas.
Powder Gangers just emerged from NCR ashes, too young for a DLC of their own.
Great Khans are the ones at RRC, no more.
BOS, no.
Boomers, isolationists who bomb everybody that comes in range, they need a DLC for?
None of the others need a DLC, Legion on the other hand does, we know nothing of how an actual Legion town/city is, how the slaves are treated, how the citizens are treated, if it's all work and no play et cetera.
And listen, it's not like you'd have to be forced to help Legion in the DLC.
If you don't like Legion then I'm sure that Obsidian can think up a storyline that allows you to help someone else take Legion down in this city or something like that.
But for us Legion fans it would really help with arguments and lore and might even make NCR, YM and/or House supporters come over to Legion's side.
Legion needs a DLC for the sheer size they're in, the amount of quests, NPC's and dialogue in vanilla was not enough.