X (formerly Twitter) has network effect lock in. Mastodon etc. never penetrated fully enough into the public consciousness for the bulk of Twitter users to move. If anything, some maintain dual profiles, but major movement from one side to the other never happened.
In some ways, Twitter was the perfect business to buy. It takes alot for users to move on, even with all its failures, flakiness etc. because at the end of the day, where else are people going to fill the need Twitter does? You can mess up all day long, do really dumb things, and keep the bulk of your users still, because honestly, where else they gonna go?
Now, it remains to be seen if Musk can turn it into a real, viable business with legs, as it where. His skeleton crew makes them vulnerable to upstarts IMO. Eventually, someone or something will become a good enough competitor that X starts losing users