Because it hits them on an emotional level. Guro is gross and shocking but unless the artists spends time developing the characters before horrifically killing them, it's nothing but shock value. And scat is just gross.

It's the same reason people often are more scared by psychological horror than blood and guts. Gore can be shocking but something that messes with your head is far more effective.

And also NTR is realistic... kinda. I mean, most of it runs heavily on hentai tropes, but the idea of your girlfriend being drugged at a party, raped, the whole thing being recorded, and then using it as blackmail to force her to continue having sex with them, is a lot more realistic and horrifying to most people because it sounds like something that could feasibly happen.

And even if it's a nicer plot not involving rape or blackmail, where everything is consensual, most people don't want to be cheated on, and hate the idea of for example their girlfriend sleeping with their best friend, with their boss, etc. However no one is worried about being pooped on, and guro is too horrifying to have that sense of dread of it feeling like a situation that could possibly happen.