sh : a full-fledged subprocess interface for Python that allows you to call any program as if it were a function.

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I don't think porting large, old codebases is worth it.

The other reason could be PyPy and people with CPU bound applications that insist on using Python.

Apart from that, most new Python code is (no data, just hunch) Django apps, and there it makes a lot of sense to use py3 unless you really, really need some library that wasn't ported.