Because there isn't a trademark conflict, it's just too generic of a term to get trademarked in the first place, but it took seven months for USPTO to process the application.
"Tesla applied for the trademark in October 2024 on the same day that it revealed the Cybercab."
Giving Tesla those trademarks would mean nobody else can call theirs using those terms. Giving both could mean they might argue the terms robocab and cybertaxi were too close and confusing too. But they are all terms we already use for that type of a vehicle.
We didn't call operating systems windows before windows, and we (mostly) still don't.
Also the trademark for X is like the trademark for Apple, very narrow in scope. You could start and trademark X as your own pretty much as long as it's something that isn't like Twitter.
And many already do exist (and did before musks X)