The protestors/rioters are less popular and mainstream than they think they are.
It's partly because, just like most people, they tend to interact with people similar to themselves.
But it's more than that. They scare people into not voicing dissent. The left in general suppresses disagreement enough that some people won't speak openly to pollsters, let alone to lefty coworkers/friends/party-goers/social-circle-members, let alone to the sort of lefties who'd join a BLM protest/riot. We saw this with Trump losing in the polls but winning the election. We also see it in China where most people say they support the CCP but if they had safe, secret ballots they'd vote the CCP out.