Because is hard to make a ntr/cuck scene properly, aka happealing.
Most of them around both erotica and h-games can be boiled down to "generic fatso/chad rape the shit out of your love/porn interest until she begs for more". This works barely (irl cuckolds states their consent) outside games, because it's left to the reader or watcher to immdesimate with whoever he or she wants, either the cuck, the bull or the bulled.
This kind of freedom isn't avaible in games since characters are assigned. You cannot pick a side you are the main character and that's it. So, depending on how the scene is played, you're forced to take one of the roles. Most people, me included, consider this a bad choice, especially if there are no hints of the upcoming scene. This happened in DoD, and obviously patreons protested.
