metalheads would rock out to Slipknot
True, but Slipknot was the most metal of the nu-metal bands.
Why was nu metal hated so much? You could argue that the style and aesthetic "hasn't aged well" today...
A lot of it was indeed kinda trashy & shitty. But also:
• not metal enough to be respected by metalheads
• not rappy enough to be respected by niggers & wiggers
• too rappy to be respected by rock fans
• too low brow, masculine, and oriented towards the frustrations of young white males to be respected by hipsters
But in retrospect, there was nothing exceptionally bad about it relative to other pop phenomena and some of it has actually aged somewhat well.