Me and Trang were talking about why this might be a while ago, and there's no clear answer, but what we came to see is this: women characters are judged differently than men characters. And that's a give, but it's how they're judged that stands out. Basically, if a female character is pretty and appealing with a nice calm attitude- fans are easily impressed with that character. But if a female character is spunkier and sometimes not so "visually" appealing- fans tend to judge that character harder. Now, sometimes these characters still gain a massive following, like Rukia or Morigana, but sometimes they become the fandom's number one hated girl- like Sakura, Chi Chi or Jericho. Then there's that mix- like Mikasa or Bulma, where half of the fandom worships her while the other half despises her.
And that's why we have bullsh*t like "best girl" instead of "best character" because female characters are on a completely different spectrum from male characters. But this is usually only found in shounen fandoms, where women characters are screwed up anyway. And it doesn't apply to everything (One Piece). But it still exists.
Sincerely, I agree with you.