This one's easy. Raceplay, at least the type that I write, comes down to the primal human fear of being conquered by another tribe. It's subjugation and discrimination eroticised. Like there's this whole trope of Orcs gangraping an Elf princess because people like it. They like seeing enemies fuck and an entire people get conquered. They like the contrasting stereotypes. There's something aesthetic to it. It's Yin and Yang.
I like using race because there's visible contrast which is hot to me, but I have seen stories online that use religion or political orientation instead. Can't remember where exactly I saw it but I came across a collection of captions made by an Indian fantasising about Hindu girls being taken by Pakistani Muslim guys. I've also seen Feminist women explicitly looking for rightwing or Muslim men to ERP with. Some people just get off to the idea of being conquered. There's not a proper name for this genre of porn. Raceplay is a misnomer IMO, misses the wider appeal of the fetish. Generally speaking if you're into raceplay you're gonna find other types of systemic discrimination hot.