Probably because they were the most prolific band of the sub-genre that took over rock after hair metal was starting to die off. I don't think they killed it, hair metal was becoming popular in the early half of the 1980s, and if you consider it an extension of glam rock even longer than that.
Grunge and hair metal aren't really all that different. Alice In Chains actually started out as a hair metal band in the late 80s.
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