The kneelers undermined their cause.
Police abuse of power is a real problem, regarding which whites and blacks should cooperate to solve. But, by making it a blacks only issue, and silencing those who wanted to stop all police abuse of power (by silincing the "all lives matter" people), these people have repeatedly hamstrung their movement. By kneeling, and offending the people they need to help them, they hamstrung their movement. It is enough to make you wonder if they actually want to solve the problem, or just exploit the problem to consolidate their hold on the African-American voting bloc.