It's a combination of multiple things, not just a single one... I'll try and point out a few things I noticed.
The antagonist stealing the MCs precious women is literally a Gary Stu (although a black one, lol). He's tall, muscular, good looking, good at sports, smart and even whispers sweet nothings and tells FMC (Kim) that he loves her. That makes for a bland NTR antagonist. I've commented more on that in other games' topics, but the NTR doesn't hit right when the dude doing the stealing is objectively better than the cuck in every conceivable way.
Tropes such as the ugly bastard, fat guy, pervy old man, etc, are a staple of NTR for a reason -- they work and contribute to spicing up the drama. NTR thrives on drama and conflict. It's way hotter when the FMC in cheating with a dude that isn't an obvious improvement over her actual partner. It's hotter when she breaks down to the point where she cheats just because of a big dick and being able to cum harder-- she cheats just for the sex, not caring if she's being treated like a cumdump.
That by itself is already a major letdown, to the point where I find one of the hotest scenes in the game is when that other random black dude showed up and put his arm over Kim's shoulder. Why? Because he actually felt like he'd treat her just like a piece of fuckmeat.
On another note... Something else I feel gatekeeps even more possible fans (maybe even contributors $) is the fact that the presented "endgame" for the story is both the MC and antagonist becoming a gay couple. That as well as the inclusion of trans characters and sex scenes with said characters, plus the fact that the story seems to be heading in a feminization direction for the MC. Think what you will, but the public for that is not that large, and it also turns off A LOT of people that aren't into that.
IMHO, there's just a lot of wasted opportunities here by including and excluding things.