It's all about stickiness. Twitter still has accounts people want to follow whereas other platforms don't so people can't shift or find it hard to. I've seen some prominent users move to Threads, Substack, Bluesky, Mastodon etc. but not enough and scattering to the four winds doesn't help either. If there were an actual exodus of accounts across media, news, celebrities, sports, government etc. then Twitter die on its ass. Or even if big accounts started mirroring their content across other social media services.

And these rival services should really federate. But we all know that the commercial services are loath to cooperate with each other when they want all the pie.