Because I use Typescript! Joking aside though (although, let's be honest, there is some straight-up selfishness: I use TS and if Facebook did, my life would be easier), I think adding types to Javascript is a huge win no matter how its accomplished and it just so happens that TS seems to be gaining more community support than Flow. Flow's support is almost exclusively from the React community because of Facebook. If Facebook would switch to TS, the whole community could coalesce around one standard. That would have huge benefits.