Because the different stories attract different people. With netori, most people here are moral enough not to play Mr. Steal your girl, so the best way to let people digest that they're still doing something scummy is to go the harem route and either make any other male footnotes, or make them mustache twirling villains/incompetent so that you don't feel bad for stealing their girls.
NTR is the exact opposite, lets call a spade a spade, if you're in to NTR, you have a humiliation fetish. The entire point is to lose your girl, which is one the most emasculating things ever, so why not pile on to the humiliation. That's why most NTR protagonist's tend to be classical anti-heroes, because you want them to lose.
