I think it's to maximize potential contributor base.

JS is the front-end browser language. That's its origin story and it almost certainly will always have that job, because of web browser backwards compatibility.

That's why it's so universally known / used / understood.

Once you target TypeScript you're suddenly looking at a much smaller set of devs who know it, and you're married to a language that's effectively owned by a single company (MS, to boot).

You can get decent gains in type-checking by writing JSDoc and using tsc in your build process without adding much friction for new contributors.