Protobuf is old, and has some really strange warts that are only explainable historically (for example, by reference to the behavior of C++, which used to be Protobuf's primary target).
also reinventing atom with vscode.
Flow has always been an inwardly-focused project that is for Facebook's needs first. The Flow team at FB made an explicit decision to focus more on compiler performance improvements (what Facebook's usage needed) over outside users & community.
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.