You didn't need Ocaml to use Flow, and it's not like the average Typescript/Flow dev ever looks at the source code of Typescript/Flow, let alone contributes to it.

Flow died out because it focused on things that JS devs didn't care about – speed and correctness. The latter especially came at the expense of annoyance: Flow tended to throw errors when you used code patterns with unprovable correctness, whereas Typescript happily "compiled" such patterns without checking their correctness. That was a "good enough" improvement over plain Javascript, and it easily won, being a more established community, and with more support from Microsoft than Flow ever got from Facebook.