Because the people with power, the leaders, big tech, advertisers, people with influence, and the media do not like musk. They’ll just start some new trendy site and go there.
You realize people have tried that before as well, e.g. Parler, Gab, Truth Social, etc?
You're likely right to some degree though, in so much as that there will be a group or a few who will for reasons (whether truthful or not) don't want to associate with a site owned by Musk and will try to start and create a successful site; it's not easy though - and why Musk is willing to buy a head start for ~$42 billion.
Still, an alternative wouldn't succeed.
Beyond the moderation, these sites have the problem that they are inherently set up as a sort of opposition haven -- "we're twitter for people banned from twitter!"
This results in a phenomenon Hank Green calls the "worst people problem"[0]. I'm not a huge fan of the judgemental term, but the core idea is similar to pointing out the inherent contradiction of, say, an anarchists' society. You end up with a userbase that is defined by its desire to flaunt rules and reject any normative standards.
[0]:https://threadreaderapp.com/thread/1348101443404787718.html
(somewhat related and worth a watch: Folding Ideas' 2017 video essay about YT alternatives. Alt platforms have had inherent problems even before the more recent political bent: https://www.youtube.com/watch?v=r3snVCRo_bI)