People often bring up their heavy hip-hop/rap style but I don't think that's the reason. Remember they were doing this with a song like as if it's your last over 4 years ago, with kpop being a lot smaller:
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So what is it, why are they so far ahead of the girl group competition? it almost feel like they are eclipsing the rest and not letting them grow. My best guess is their very distinctive voices, something you almost never find on any kpop group