[mmv]

The way I remember, when flow was introduced, you had to do small changes to 3rd party dependencies your code might have had to make them flow-compatible.

With typescript you could just write a type definition file for any 3rd party library, so you could essentially make any 3rd party dependency "typescript compatible" without needing to change its code.

This small difference made a huge impact for adoption. Eventually flow also got that feature but by then the adoption difference was already too big.