Because my city deems it is still a good idea to post important announcements about roadworks and water interruptions/boil advisory there exclusively :/
With the small carve out exception of people who are fine with the antisocial behavior and management style of the owner, Inertia is the only reason. Governments, influencers, corporations- they all have a following and an established channel which involves minimum expense to maintain.
I'd say that the vast majority of people on Twitter couldn't care less about Elon Musk himself. However, Twitter is actually going downhill now and the only ads I see anymore are either hardcore porn or scams. That's what's going to drive people off Twitter.
You can only view a single tweet at a time when not logged in, which can be a struggle if youre trying to just see updates in general and aren't seeking a specific post.
I know because I have a friend who still sends me links to make them useful I have to put them in Nitter.