The other route destroys a game with a vanilla route for most vanilla players especially if same character is in it, since people always find out what happens in other route even if do not play it. Even if just by players posting about it that are enthusiastic about that content.
Now if you do not see it as same char,it does not matter or just want sex scenes with them, for many in the more vanilla crowd that ruins their game though so they will not support a game with both story lines and another line means a lot of dev attention goes to a line they do not care for with tens of games there without that other line as well that are fully fun for them. Why would they pay for the development of that NTR line they do not like and play and ruins game for them?
If want vanilla money make a vanilla game, if want the fetish money make the fetish game. Betting on both horses with two such differing needs of players worked in past when market was way less saturated, but not anymore. Too much choice.