My theory is based on the 1:9:90 internet "rule", meaning that you have a core group of people, typically 1% of the users that are highly active, some further 9% that post & comment occasionally and the vast majority of 90% only lurks (but might share a link in their social circles).
Taking that into account, I think why some sites fail and others not is mainly about the 9% occasional posters. If the 1% hard-core users (due to ideology or lack of moderation) manage to alienate these "casual" users then the site goes into a death-spiral that is hard to recover from.
So besides stricter moderation, I think the hard-core users of more left leaning communities are also typically less offensive/toxic then those of some other communities who often thrive on 4chan like meming and in-jokes that really only work for the hard-core user-base.