OK, I'm a little drunk and already realize that this will probably not be a well received post. I'm sure some of you have already closed this thread, but hear me out.
Nu-metal drew heavy inspiration from hip hop, which I remember many back in the day naming as one of the reasons for hating it. Sure, there are so many terrible white boy rappers in the genre, but as we know many nerds have a burning hatred of rap to this day, and it's almost certainly at least partially thinly veiled racism.
A band having a DJ was definitely singled out as a reason to dismiss them, and since DJs grew out of the hip hop scene, it reeks of 
to me.
You could also argue that it's just narrow minded rock and metal enjoyers dismissing anything outside of their genre, but I definitely don't remember them having nearly as much of a problem with industrial bands like NIN, Ministry, Godflesh etc. using samples, drum machines and electronic music elements. It sure seems like hip hop influences were far less tolerated 
Am I onto something or is this just weird overthinking that randomly came to my mind?
That's funny, considering Thrash Metal/Speed Metal is basically just taking Iron Maiden and adding punk tempos from bands like Bad Brains, Black Flag, and Dead Kennedys. By the above definition, Metallica is Metalcore.
Slayer even released an album where they cover a bunch of punk bands.