the verge is reporting that he's being tried as an adult, so maybe that's the reasoning.
https://www.theverge.com/2020/7/31/21349920/twitter-hack-arr...
>He’s being charged as an adult, and the press conference made clear that law enforcement is considering how bad consequences of the hack could have been — not just the $100,000-plus in bitcoin that the teen is alleged to have scammed out of unsuspecting Twitter users.