Flow and ReasonML are started by completely different people/teams inside the company. From what I heard, Facebook the organization does not care that much about OSS, and software devs have to work really hard to convince to spend company time to maintain an OSS software.
If there is any battle, I predict will probably be between Flow and Reason inside the Facebook org because of competing solutions.
They lost the Flow-TypeScript battle. And they learned, they wanted even more typesafety I guess.
Since TypeScript also compiles to JS, bringing "something a little less like JS, but a lot more safe" to the table seemed a better idea that pushing "unfamiliar syntax, but heay, it compiles to JS"