I like NTR, but I don't get the obsession with the race thing. I really don't understand it.
What's not to understand?? The biggest selling point for NTR is the corruption and degradation of a love interest. So with that in mind, it should be fairly simple to understand why the person doing the cucking, regardless of race, is often depicted as a lowly, poor, out of shape man from the dregs of society. As often, or potentially even more often however you see them depicted as a city official, royalty, powerful heroes or knights, or well to do merchant, athlete, or scholar, or something all of which with extreme sexual fetishes - these are used to show how useless the protagonist is in terms of capability, their lack of upward mobility, lack of wealth, etc.
So then an added layer to all of that, is race. Whether you like it or not, the vast majority of people seek relationships with those of the same race as them, or at least a similar skin color. So the plot of having that love interest stolen by someone of another race, is used as an added level of degradation, both towards the love interest, and the player character. There's multiple factors here, most simply it tramples on the ideal of the characters involved, who would typically seek out someone of the same race as them, then finding out that they are actually interested in someone of another race, or they are being forced into a relationship with someone they wouldn't normally pursue, or it could be done for blatantly racist reasons: i.e. a racist watching their love interest being stolen by someone of another race.
So again, I don't really see what you don't understand here, unless you're just wholly unfamiliar with what NTR is supposed to be at its core.
