They were hoping that twitter would go the way of Digg. It's very similar to the fantasy that they had about Reddit after the API shutdown. Slacktivism.
The fact that twitter is as active as ever is why they slink back. I'd bet 90% of actives on bluesky also have twitter accounts that are more active. Twitter didn't care when they walked away; in fact it was a relief. A great many of them were extreme trolls and reply guys who spent the rest of their time mass-reporting people for disagreeing with them.
And for the few people who thought of this as the p2p answer to twitter (an obsession among programming p2p nerds - including me - for a long time) it has to be a disappointment that the company obviously has no intention of relinquishing control of a thing.