I’ll just pass along what I learned as a white kid, who loved rap, in a mostly black high school. This is all out of date, things are “better” now, but some history probably helps here.
It’s basically like basketball. Some white guys can be good at basketball, but they’re expected to be John Stockton or Larry Bird in the short shorts, even today. Technically proficient to cover up their lack of athletics, which is “true” talent. I played as much basketball as I could, there were consistently one or two white guys, so we were either “white boy” or “Opie” if someone was feeling good. It’s a lot of fun to dunk on the white guy, it’s extra shameful to have the white guy block your shot. Today there are 2 non-European white guys in the top 50 here. There’s a racial element that white basketball players don’t pass the “eyeball test” (get em Gordon).
Before Eminem, white rappers were mostly pretty bad. Not that there aren’t a lot of bad black rappers, but the most famous white guys would have been something like Vanilla Ice, Snow, and House of Pain… that’s really it before Eminem. (honorable mention to Marky Mark, but he was someone’s brother, so it was always assumed he’d join a boy band soon). The Beastie Boys didn’t count, because that was something different.
Good producers would find a white guy who was OK, make some hits, and then that guy would disappear, because it was seen as a gimmick. (see Sparxxx, Bubba)
So most white rappers are viewed as gimmicks. They’re here for an album, maybe two, which each have one hit, maybe two, then gone. They’re seen as having outsized success compared to their talent, like most producer driven musical artists.
Eminem is different because he’s actually really really good. You can disagree with his content, his message, or whatever you’d like, but he’s good at both rapping and marketing, and has been for a long long time.
Compare that to Post Malone, the latest and greatest white rapper. He’s captured a moment, but it’s hard to believe he’ll be anywhere in public consciousness in 5 years.
Nicki Minaj has probably seen enough to know that, and has had some success going after the short timers before. Gotta stay in the public eye somehow, I guess.