I think it's because female characters are more often the target of hate than male characters, in general, so some people want to get away from that "culture" of negativity that surrounds female characters. Sometimes it's because female characters are held to a standard that is too high. Then other times, I'd say it's because people either don't want to be "that fan" who "hates all female characters," so they're defensive of them instead. Sometimes people are defensive of female characters in general because they believe that writers will only start writing quality characters when they perceive that female characters in general are received positively.