Because Go is at least as fast as PyPy or Cython, have a similar coding feeling to Python, have language level concurrency support, and has a clear/consistent development roadmap moving forward.
PyPy is not really a good example here. It is still highly experimental, takes long time to compile, it's not compatible with quite a few libraries in normal Python, and still has all relative downsides of dynamic typing. To be frank, I don't get the point of using PyPy instead of Go.
Previously most Python libraries already use C to optimize critical components. If you can use just one language to get similar performance, why do you bother using two then?
The issue is not about writing clean new programs in Go, but rather re-writing whole code bases from Python into Go, instead of trying out Python implementations that offer better performance than CPython.
PyPy is not really a good example here. It is still highly experimental, takes long time to compile, it's not compatible with quite a few libraries in normal Python, and still has all relative downsides of dynamic typing. To be frank, I don't get the point of using PyPy instead of Go.
Previously most Python libraries already use C to optimize critical components. If you can use just one language to get similar performance, why do you bother using two then?