Any platform that establishes itself as a result of a purge from a left-wing dominated platform like Reddit will start out as explicitly right-wing and will struggle to gain non-partisan membership. That new platform will get its reputation tainted for being a place for extremism - even if it permits both left-wing and right-wing views. The left-wing folks won't see a need to join the new platform because they already have the existing one to feel comfortable in. They are content to let the new platform be seen as a hotbed for right-wing radicalism, continue to smear its reputation, and then wait for it to whither and die.
On the flip side, any platform that starts out as non-partisan eventually gets dominated by left-wing / left-leaning folks. I personally have seen this happen countless times on subreddits individually, then reddit as a whole. I sense that this is because activists on the left tend to naturally gravitate towards becoming moderators and establishing new forum rules whenever they take over. Most of the new rules are unobjectionable but questionably appropriate. "No Racism / Homophobia / Transphobia" is a fine rule, but is it really necessary in a woodworking subreddit? The other kind of rules are more subtle and easy for moderators to abuse, like "be friendly", which can be applied against right leaning folks and ignored when left leaning folks get confrontational. Eventually the conservative/right folks get drowned out by agenda-posting and getting downvoted en-masse. They either self-censor, just accept the downvotes and keep posting, get banned, or try to leave for a new forum.