This is just my theory but I'd say it's because of how things are nowadays (NTR wasn't as popular before after all). You have plenty of girls who play with men, far more than before, with a vast majority of divorces initiated by them so they can get child support. Paternity fraud is more widespread than most would think. Before hitting the wall at 35-40 they date the players and after that age "no more fun, I want someone serious"... meaning no fun for the responsible, caring, loving beta while the alphas all had their turn before. Around 25% of girls dating on Tinders are doing it for free lunch or to have the guy fix something at home. Etc etc. In other words, plenty of heartaches... and those players also break hearts, make no mistake.
So now when those guys who were hurt or heard of such deals see an NTR game (not saying all NTR loving people, guys or gals, are like that), they're like "hah, another supposedly sweet and lovey-dovey girlfriend who happens to be a lying, cheating gardening tool" and get off on her suffering because she's getting her just desserts. The more douchebag or uglier the dude, the better. She doesn't deserve happiness in her life. Sugarcoat trash all you want, it's still trash in the end.
I may be wrong, but I found that makes sense. I'm not into NTR myself though so take that with a grain of salt.