Didn't they immediately ban JD Vance on his first completely innocuous post on the platform?

That's aside from the blatant echo chamber nature of the platform. Both the left and the right post on Twitter. Only the left post on BlueSky.

This probably contributes to the decline, but I think the biggest contributor is the lack of interesting content. If you wanted to gorge on leftist viewpoints wouldn't you just read a leftist subreddit?