I think that if you have a neutral social network and a politically-charged social network, the neutral one will attract more eyeballs. People like to see ideas challenged and debated. Heavily-moderated and single-sided networks (Mastodon, Truth social, etc.) are simply boring compared to celebrity drama and political clashes on twitter.
What you have in the modern internet is that left-leaning users avoid networks that aren't moderated in their favor (in a conscious attempt to prevent moving the overton window), which leads to right-wing takeover (and eventually death of the social network because there are only right-wingers, see the graveyard of reddit alternatives). This trend would have been reversed a decade ago.