I don't think I've ever seen it explicitly stated what the criteria are.
My guess would would be some combination of character/personality, having a substantial AV career, and a general consensus that it's a good choice.
I suppose DMM, the production staff, and the girls would be the main players in that consensus.
Previous leaders were Sora Aoi, Yuma Asami, and Kishi Aino.
IMO, behind all the cutesy stuff, Sora is a tough cookie with real leadership skills (same with Aino).
Yuma always seemed a surprise choice for leader...too soft/nice. But, who knows, perhaps, in reality, she is just as tough as Sora.
Or, maybe, Yuma is more the 'soft power' type. Gets things done with a smile and a kind word.
It will be interesting to see who they pick.