To not support it's new owner
But what do you know about the owners of the Mastodon instance you registered your account with? Are they accountable? Are they sensible? Are they competent? Do they even have a real identity?
None of this is true for Twitter.
So yeah, you can go to another instance, but better keep some kind of backup because you could be gone without notice
Isn't that one of the reasons for decentralization? Less power and data for a single instance.
So yeah, Musk gets investigated very often so we know things that he would rather keep hidden. Now what about your Mastodon admin with an anime profile picture?