For the same reason we often let a small group of activists decide which statues to erect in the first place?

Many changes which I'd expect are commonly understood to be positive for society started with a small group of activists. Movements start small and public opinion doesn't change overnight. Sometimes public opinion needs a nudge from a small noteworthy action.

Had you ever discussed applying democratic values to the evaluation of the suitability of historical public statues spontaneously, before these actions made the news?

Indeed. Have a popular vote.