Because many of those involved in the pastimes (anime, manga, and everything related; I also mean creators, not just the fans) are effeminate and thus have some kind of longing for submissiveness--from which comes their fetish for rejection. From JP video games in which you can wax lewd with a (favorite) girl and still hear her treating you like a disgusting pervert in response (
Senran Kagura, of all games, did this) to their love for catty 99-100% tsuntsun tsundere girls to
many things I'm leaving out, all of these are based on a desire to be looked down upon--to be underfoot.
Naturally, one of those aforementioned things is NTR--they like the feeling of being a helpless cuckold.
It's not just the Japanese, though: how many people around here can you see calling female characters "mommy", voicing their willingness to be harmed by vicious females as long as they're sexualized in some way, or otherwise expressing a desire to be dominated by a given female character? The essence is the same, which means the market's a lot bigger for this kind of depravity than one might initially imagine--if both Japanese
and outsiders are this pathetic.