It's companies. Corporate PR accounts never migrate off to Mastodon/Bluesky/Threads. Many neglects update of updates pages, announcing everything on Twitter.

Some of them would move to Instagram if it makes sense to do so, but Twitter apparently makes the best ad ROI figures among Twitter-like services.

a few companies (mainly those in the open-source space) run their own Mastodon instances, though this is more common for FOSS organisations