[drschwabe]

One theory: TypeScript

Maybe tangential, but I have seen a pattern of projects that go all-in on TypeScript only to become overly complex & lose development steam. I'm not saying that is or will be the case with Codemirror but I do think there is some merit to the idea that JavaScript helps developers code effectively at the high-level whereas TypeScript has a way of taxing that process; detracting from what made the original project come together in the first place.

Of course, there are plenty of projects that have done exceptionally well after going all-in on TypeScript (or from the get-go) so take this with a grain of salt.