the activists/radicals on the left are overwhelmingly young folks who push for things they dont even show up to vote for (especially in primaries where their candidates need them most). so they hang anchors around the neck of the party mainstream that they hardly help to lift up electorally.

bernie ran a campaign specifically targeted to that part of the base...and his run was virtually/functionally over on super tuesday. the very 1st day that multiple states voted.

the activists/radicals on the right are mostly social/cultural reactionary boomers who can take extreme positions and not suffer much electorally because they show up and vote like clockwork...presidential, midterms, primaries, generals. so the politicians they pressure to support these policies can always count on their backing once it comes time to defend the policies and get elected.