Good question [I am the 'Pieter' on that blog post]. There will be a talk at PyCon that covers parts of this. It comes down to two reasons:

1) Performance. We needed something that would consistently work quickly on Instagram's server codebase (currently at several million lines).

2) We are building deeper semantic static analysis tools on top of Pyre. We've built some of these tools for Hack/PHP already, so following the Hack type checker's architecture is the best way for us to achieve this.