Because Mastodon and the entire Fediverse is infested with the most rabid sort of users that even Twitter couldn't stomach and has turned a blind eye to this fact? I mean, there's a reason why they keep getting brought up over and over and people continually nope out of signing up over there. They've got a demographic problem, they refuse to acknowledge it, and it's killing growth. Why would any media outlet want to participate knowing that?
If you think everyone on X is a "nazi" then clearly you've got problems. Twitter appeals to a massive audience, whereas alternatives do not. Media outlets will go where the eyeballs are... and that's not to Bluesky, to Mastodon, to the Fediverse... but to Twitter... which is populated by people, not nazis.