I think they lean harder on V-tubers because they use animated avatars and as we all know cartoons are for kids 🙃

a man wearing an apron that says hudsucker industries holds up a piece of paper

ALT: a man wearing an apron that says hudsucker industries holds up a piece of paper

Twitch's bias is probably a result of that and the bizarre hangup some Western companies have around anything related to Japan (anime, manga, video games from Japanese devs, etc.).