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Nu-metal reappraisal I think came from not just nostalgia, but from people not just stopping at the surface level "What the fuck are you crying about?" critique that a lot of anti-nu-metal had back in the day. Not to mention the reappraisal is of the more prominent bands rather than the genre as a whole, which suffered from cheap not!Korns and not!LimpBizkits. I would also add certain things like people coming around on the more "mainstream" ones like Linkin Park now that a lot of modern metalcore and hardcore bands taking some inspiration from them.
As for why buttrock never got the same treatment, I think it is mainly because it's easy to hate buttrock. Nu-metal at its' worst can be cringy, buttrock at it's worst is downright embarrassing. Not to mention that things that get classified as nu-metal can have distinct sounds from one another, while buttrock usually sounds the same tonally, which makes it so much easier to make broad strokes even with the notable bands.
As for staind specifically, that's because mainstream opinion on Aaron Lewis got worse. I'm not going to state mine, because mine has no effect on the greater public opinion.