He fits a very specific urge/demand of white consumption: the great white hope. The idea that no matter what black people are doing, somewhere in the universe there's some random white guy who can do it better...and white people will line up a mile away just to marvel at it. At least Eminem was a great rapper at one point. At least Bill Evans was immensely talented and one of the greatest musicians ever. But this fukking dude? Yea he has some cool melodies but come on.
The same people who will tell you that Future doesn't make real music will tell you Post Malone is a genius. That's the problem I have with it. If you're gonna fukk with Post Malone you should fukk with people with as good or better melodies. Thug, Future, etc. But these white people wouldn't be caught dead listened to those guys.
Post also can play guitar so that gives him even more cred with that crowd. BTW this isn't all white people. Shout outs to the ratchet white girls who heavily fukk with Kevin Gates and think Post Malone makes trash pop music. I'm talking about the "well I don't listen to rap but Post Malone and Eminem are definitely the best ever" white crowd.