I think (2) is the overriding concern: we're getting really great results with the static analysis tools we've built internally on top of the Hack type checker.

I think (2) is the overriding concern: we're getting really great results with the static analysis tools we've built internally on top of the Hack type checker. Building a similar tool on top of mypy would've required fairly invasive changes; we decided to use the Hack type checker infra instead. Full disclosure: I worked on the Hack type checker briefly, a long time ago :)