idk know about ntr nts fans, i just really like corruption myelf. but i think people who dont come for romance want a love route cause without it, everyone just sucks. often stories start and say, this couple loves each other, buuuuut, and then the player/reader gets to see everything fall apart, see the MC be an ass or a wuss, the FMC a whore or a bitch, and thats it. one is left thinking, they obviously dont love each other, why are they still together, why did they even get married.
when you add a love route, it allows people to see, okay their relationship is real, there was something there to protect, to hold on to. and thus when they open the relationship or destroy it completely, it has an actual effect. this could also work by giving a lots of time and energy into backstory, but everyone wants the now story. having multiple routes to compare actually makes the relationship more rounded, when done right. cause sometimes the characters behave so drastically different they arent even the same people anymore. just same face and names.
but this is all based on my opinion and joy of corruption in all its forms. love routes give a foundation to corrupt. i think a lot of people are drawn to this aspect whether theyve reflected on it or not. but i could be wrong. as ive said, i actually dont know what drives ntr and nts fans. i simply read their comments along with everyone elses and get an inkling. i tend to make an assumption out of myself though.