[jitl]

More generally Microsoft treated Typescript as a product - they built it based on feedback from outside users and worked hard to make it easily adoptable and to grow a community around it. They are also "pragmatic" - willing to have weird, unsound semantics if it's needed to help people adopt Typescript and model existing JS ecosystem patterns.

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.