It was the single platform where scientists and the politically active could interact, safe from regressives via twitters moderation policy. Essentially, a place where verified experts could share opinions to a receptive audience.
By taking over, the verifiablitiy went, the voices of the uninformed were weighted the same as the informed, and whatever (mostly left-leaning) communities existed there became fragmented as the twitter experience went down as the UX became more hostile and ad-ridden.
The progressives are recovering, and finding their communities again, but mostly via bluesky/mastodon