Because it is simplistic, Cookie cutter bullshit.
Here, go to literotica and look up "Another Love", "February Sucks"(Both the original and the better version by Cockatto),"The Bridge", "A Joke" by agena and the follow up stories based on it, etc.
They are all NTR, thankfully on Literotica people (who hate NTR) can write better endings for them, but they are so popular because the stories are original, and stay away from some of the more obvious cliches of ntr stories.
Here is an idea, if your ntr story would not be put of place in the thousands of trashy, vomit inducing ntr hentai released every year, then the story and idea behind it are trash.