[nuclear_eclipse]

The easy answer is C extensions. PyPy does not support C extensions, and thereby does not support a wide range of native, accelerated libraries. Facebook infra is heavily dependent on wrapping C/C++ libraries (like Thrift) for use in many languages, and not having C extensions (or Cython) would cut off a large portion of our shared codebase.

[Doxin]

Pypy does support C extensions, but it has to somewhat emulate the cpython API so it's rather slow and doesn't work for 100% of modules.

If your C library uses FFI or Ctypes it'll work perfectly fine under pypy.