[Mehdi2277]

Pyright is pretty recent and started development years after mypy. But some of pyright's design goals (laziness for LSP) would have made it very difficult to do in mypy without a very large refactor/rewrite. Pyright is implemented in typescript for better LSP/vscode integration and performance. pytype (google's type checker) is only one I know that predates mypy for when it started development.

Pyre's reason is interesting, https://github.com/facebook/pyre-check/issues/38. Pyre is based off of Hack, facebook's php compiler. Facebook made pyre to re-use tooling/techniques from Hack on python. Secondary reason is performance. Pyre is done in OCaml and having same performance in type checker implemented within python is hard.