That's because He-Man characters require a He-Man voice, and most male VTubers I imagine sound like the androgynous twinky types (correct me if I'm wrong, because I don't follow the scene and have no idea if this is actually true).

For women, so long as they don't sound like a complete lard arse, they can get away with a cute avatar. For a guy, if he has a cool, buff-looking bloke as his avatar, you're at least half expecting him to sound as cool as he looks. If he sounds kind of nerdy/twinky, then it completely ruins the visage.

Again, I could be wrong -- and maybe the male VTubers sound like Ray Winstone or Kevin Grevioux and just decided to stick with twinky avatars. But if they did sound like Winstone or Grevioux, then by all means they SHOULD be using those He-Man avatars to full effect.