As far as I can tell, it's because it's not based on CPython. For whatever reason, to GvR Python == CPython, other interpreters don't matter. It's a shame, because python the language is fine, but CPython the interpreter is janky and slow.
PyPy is a trade off and not universally better. It has a larger memory footprint (already a bit of an issue for Python), higher startup times and incomplete compatibility, in particular with C compiled libs used precisely where performance matters.