It's one of the ultimate taboos. We're taught that men are supposed to be dominant, "alpha" studs and take care of everything our women could ever need. Every material need, every emotional need, everything. And of course we're supposed to fuck them like pornstars 6 nights a week with our redwood-sized cocks.
And our women, especially our wives, mothers, and daughters, are supposed to be chaste and devoted to us and love us to the blind exclusion of all others.
So NTR/cuck porn hits two massive, fundamental cultural taboos right in the heart. The man failing to be Mr. SuperAlphaStud, esquire, and the woman giving away what rightfully belongs to him.
As long as we define masculinity and femininity and the relationships between them the way we do, NTR will be an inescapable fetish because it is so far beyond the expected norms. Doesn't mean any particular reader or game player actually wants it to happen in their real life (not that there's anything wrong with it if they do) any more than all the incest games mean everyone really wants to fuck their families. It's just a very taboo fantasy that draws people that respond to very taboo fantasies.