Political protest against police violence didn't peter out in 2020 because police used violence against political protesters.
Political protest against police violence petered out in 2020 because white liberals got the electoral result they wanted.
White people were never really protesting against police violence. They were protesting against their loathing of a political figure.
They'd tried repeatedly to mobilize such protest themselves, and failed.
So when Black folk (incl. Black trans women) mobilized sustained resistance, white folk leapt aboard, seeing a platform for their own agenda.
Once that agenda was achieved, they showed the same continued disregard for police violence as they had prior to the uprising.
Police violence didn't take the wind out of the sails of political protest against police violence. That wind was only ever fickle and inconstant to begin with.
That wind was white people, blowing hot—until they got what they wanted and went back to cool indifference.