And yet the popular social media sites have far more moderation than classical platforms, which are not regarded as causing problem. You mean low-moderation platforms tend to have objectionable content. True, but I don't see the problem.
Which are the classical platforms? I'm not sure we've had any instances of wide reach distribution systems that completely lacks editorial oversight before internet companies figures out how to monetise raw traffic.
Part of it is popularity/reach. People find objectionable content more objectionable when it's more visible.