[quote=Innue]The anime threads do well because there is a higher demand and audience for them.[/quote]

I know anime is popular, but it ain't that popular. Unless if you hunt down an anime club people will generally just know something about Naruto, Bleach or One Piece.
Maybe Death Note and Attack on Titan, the interest is there but minor. Cause they're more invested in say Breaking Bad and Game of Thrones.

But super heroes? Almost everyone has seen and likes Batman, Superman and Spiderman. And Marvel is exploding in popularity with their Avengers movies, the individual movies of each of those heros, and with all the X-Men movies. You could get a big discussion with almost everyone about superheroes, but not with nearly as many people on anime unless if it's "That anime girl's hot. She has big tits".
And I say this as someone who does enjoy a lot of anime myself, it just is not nearly as prevalent or popular in everyday life unless if you consciously surround yourself with anime fans.

[quote=Innue]Anime is also very, very general, while superheroes as a genre is a bit more narrow and focused.[/quote]

Only if you flat out go "Anime RP". But the majority of anime RP's are based on a certain anime like Naruto or Bleach (picking mainstream ones here).
While superhero's generally let you make any hero, they have to. Outside of Justice League or Shield it would essentially be "You be batman, everyone else be cops or villains" sort of deal. So by allowing people to create basically any hero they want, it adds are more generality and variety than most anime RP's would.