It's more that the "far left wing cluster" had something like a "we should all get up and leave Twitter for BlueSky" activist campaign. And "far right wing cluster" didn't.

The closest thing "far right" had to that was Gab and Truth Social, and that's both more specific and less impactful overall.

Thus, BlueSky's userbase is biased towards extreme left wing - it's basically the go-to place for far left wing nutjobs go when they get too nutty for Twitter moderation, or feel like Twitter is not left wing enough for them.