He’s kind of said it himself on multiple occasions.

“Twitter as a company has always been my sole issue and my biggest regret. It has been owned by Wall Street and the ad model. Taking it back from Wall Street is the correct first step.”

https://mobile.twitter.com/jack/status/1518772754782187520

He will likely come back.

Yeah - it wouldn't surprise me if Elon eventually puts Jack back in as CEO.

He had only a tiny amount of company ownership, didn't have total voting power, the board (which barely even used Twitter) made it impossible for him to make the changes he'd want to make and just generally seemed obstructionist/bad. Jack has talked a lot about how making Twitter a company (instead of a protocol) was a mistake, he's talked about Web5 which lands on a lot of the same ideas as Urbit, but trying to tack it on top of the existing stack. He supported Elon taking power and being able to actually make the necessary changes and recently tweeted support of the layoffs too.

Pre IPO Twitter was doing interesting things with API access and being more protocol like, but they killed all of that stuffed ads on it and went public. Ben Thompson talks a lot about how they're a terrible ad product and I think he's generally right. Subscriptions are a more interesting model.

If you read the texts between Jack and Elon that came out during the recent legal fights over the sale you can see these things for yourself.

That's somewhat of a rambling answer, but a lot of this stuff is publicly verifiable.