Just cause he has a female voice. There's only one Mewtwo..thats like the whole point of his character. Do people realize he uses telepathy and not vocal chords. So he can talk in any damn voice he pleases. Also think it has something to do with this X aspect. Eclair Forme could be feminine and the Y forme more masculine and muscular.
PS. I do wish Eclair was pink though. Would channel Majin Boo even more.