I'm not sure that it was just Facebook-specific needs. Aside from being faster (for their large code size), back in like 2015 Flow was also a better type checke

I'm not sure that it was just Facebook-specific needs. Aside from being faster (for their large code size), back in like 2015 Flow was also a better type checker than Typescript despite being a younger project. It made different design decisions, focusing on better correctness, at the expense of some convenience. Typescript "won" because it was actually very loose, especially back then, especially with default configuration. It happily compiled heaps of incorrect code patterns without any warnings. Many JS devs had no prior experience with statically typed languages, and they saw it as an improvement over the status quo of plain JS, and didn't care enough about slightly better but newer and thus less popular alternatives like Flow. The frontend devs who expected soundness and correctness from a statically typed language were (and still are, by the looks of it) in a very small minority.