Big mom is the only female yonko
Boa hancock is the only female shichibukai
Its their status, power and potential
Yes robin has so much potential but what she already did thenn?? Even in rencent chapter she doesnt have a pants for a god sake
So you're mad that the yonko and warlords aren't 50/50 male and female? What makes you think they should be? Are the top professional fighters 50/50 m/f? Are the world's militaries 50/50? No they're not, it's a simple fact that men and women are built differently, men are generally stronger, there are some women that can achieve that same level but they are the minority, it's nowhere close to 50/50.
With that said how does it make any difference how robin is dressed in the recent chapters? Her outfit doesn't in any way effect her level of strength, intellect, or overall importance to finding the one piece.
Hopefully someday, after you lose your virginity, you'll be able to see past a couple cm of leg skin on a manga panel and be able to notice the bigger picture.