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?
I never thought of it that way when it came to nu metal hate, but metalheads just, in general, tend to be elitists and purists who think that any blending of their beloved metal with another genre of music is bad. It's why a lot of metalheads feel the need to not shut the fuck up about how much they hate metalcore, for instance. Their opposition to it usually stems from not seeing it as "true metal" and just a subgenre of hardcore punk that incorporates metallic elements. Regardless if you hold that sentiment as truth or not, just because a form of music is "not metal" doesn't necessarily mean it's bad. You also have to consider that the lines between a genre can definitely be arbitrary and socially constructed.
There are also a fair amount of racist metalheads.
Regardless, metalhead purism and metalhead racism are both toxic traits.
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Slayer even released an album where they cover a bunch of punk bands.
Coming from a different angle, I was deep into the punk rock scene at the time of the nu-metal explosion and we disliked the genre because we viewed them as corny fratboy meatheads. I loved both metal and hip-hop but despised nu-metal bands even though I enjoyed some proto-nu-metal bands that fused the sounds (like Anthrax). And the Judgment Night soundtrack went HARD. So I guess some of it was just prejudiced elitism on my part.