I was a hip hop head when he came out but NOT on the subreddit so take it with a grain of salt. I think it was two things:
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The rapping is what makes it "hip hop" but it's basically a different genre. Not nearly as different than Bare naked Ladies' rap sections but still very different. And his music is (was?) pretty mid by hip hop standards. Trying to compare mackelmore to one of their darlings like MF DOOM is just...not fair if hip hop is your only criteria.
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The framing around his rise to fame was so, so bad. No fault of his own but you had white guys saying shit like, "I dont like rap but this mackelmore guy is great" and the more racist "this is how poor white people act" (in reference to his breakout single). The media sure didn't help, the fact that he was white was somehow noteworthy.
So r/hiphopheads had the whole world telling them that this Macklemore guy is the greatest rapper of our generation and they reacted hyperbolically.
I get it. It's like everyone telling a LOTR fan that Star Wars is the greatest fantasy story of all time....actually that might be too much of a stretch but hopefully you get my point.
I wrote this while listening to Aesop Rock FWIW