Dart is something very similar to Java, not much of a bet to jump into it. Moreover, it has type soundness, something Typescript cannot achieve.
I'm not so sure it would be easy to continue RN core development either. From what I see even just upgrading the JS engine from an ancient version was a major hassle for the core team.
In RN there's simply more moving parts. You work with 3 package managers at the same time, possibly even 3 languages. At least at this stage Flutter just seems overall more productive for the developer.