The left-leaning vibe is a historical accident reinforced by user-managed moderation.
Bluesky came at the right moment to pick up lots of people fleeing Twitter after Musk's overbearing edgelord enshittification of Twitter.
Now, Bluesky's robust moderation tools allow users to subscribe to user-curated block lists. Users are empowered to decide they don't want to hear certain viewpoints. You don't want to see cat videos? Subscribe to a block list.
Right wing folks mostly don't care to try Bluesky because they have Twitter, but those that do try don't get much traction because no one sees their posts. Trolling and rage-baiting become unsatisfying when you're talking to yourself.
As a brand new user, I see a lot of posts obscured by moderation. What I'm saying is that bsky is not only block lists: there's moderation applied without opting in to it, right off the bat. I created a new user just to try it, I went to the Babylon Bee's bsky account and all its posts are covered by a banner right out of the box.
On the one hand there's the Bluesky moderation service. By default if you use the official Bluesky apps/bsky.app experience (the Bluesky AppView) you are automatically subscribed to moderation decisions by the official moderation service.
On the other hand, you have user-led behavior. One of them is blocklists. There are large blocklists run by individuals or groups that try to keep track of right leaning accounts that many users subscribe to in order to shut those voices out. As is easily observed, there's lots of conflict as to why a given person ended up on the list. On top of that there's a general practice on the network where users pile onto perceived ideological opponents by hate posting at them, occasionally with death threats. This ends up pushing away most of the right oriented users and continues to enforce ideological rigidity on Blueksy.
The asymmetrical social dynamics of the network are largely what create the ideological bias you observe. From what I can tell the core team's focus has been moving away from Blueksy and more to the ATProto layer itself. My thoughts are that Bluesky itself is probably a lost cause from an ideological melting pot perspective and that you'll need a new ATProto app for that, but this raises the question you asked, why not Mastodon? (The alternative could be to have the core team bring on some celebrities and pay them to post but due to the nature of the existing community I'm not sure that'll go super well.)
Bluesky came at the right moment to pick up lots of people fleeing Twitter after Musk's overbearing edgelord enshittification of Twitter.
Now, Bluesky's robust moderation tools allow users to subscribe to user-curated block lists. Users are empowered to decide they don't want to hear certain viewpoints. You don't want to see cat videos? Subscribe to a block list.
Right wing folks mostly don't care to try Bluesky because they have Twitter, but those that do try don't get much traction because no one sees their posts. Trolling and rage-baiting become unsatisfying when you're talking to yourself.