Gab doesn’t federate, so isn’t really part of the fediverse. They’re just using the software; they’re their own thing, not part of a distributed social network. One could make an atproto island that didn’t federate, I suppose, but it would be a lot of work for no obvious reason.

(There are a few other far-right mastodon instances, but then there are pockets of the far-right on bsky, too)

Isn't the answer kind of obvious - much more weight of the developers in comparison to the random people from open source community. Also timing: Mastodon and ActivityPub appeared in the tail end/transitional period when there was still some common public square mindset (at least among random people not radicalized in any way), and tech inclined people had protocols vs. centrally managed platforms in relatively fresh memory. So in the mid-2010s you'd get all sorts of people flocking to the promise of free internet.

When Bluesky was taking traction, the cultural expectation among its audience was already for the platform to heavily shape the narrative. Paradoxically, AFAIK the Bluesky devs themselves are pretty serious about it being an open standard, though I'm basing this on what I heard. So I mostly believe people that the echo chambering you mention is structured in a way that it's technically not centralized. Though in practice, it's way easier to amplify left wing messages on closed websites like YouTube, Facebook, X (I'm basing this on algorithmic recommendations I'm getting and experience of people I know) than the other way around on Bluesky. But this is just the weight distribution of the audience.

Even then non-left supervised Mastodon ecosystem is something of a deep cut. I mean you're right it exists and now I recall hearing about some drama years ago, but not a part of the front and center info about them, for any common person. So I'm not fully buying the contrast you're trying to build here.

"Why is that?"

Elon Musk took over Twitter and made it rabidly right-wing. As a result, a lot of left-wing people went to Blue Sky.

Social networks are just groups of people: if such a network starts with a core group that leans heavily on a particular end of the political spectrum, the network will also (even with zero bias from the network itself) lean that way.