When an interpreted language breaks backwards compatibility, everyone is affected. Users suddenly can't run the program when their interpreter is updated.
Which is why we have separate
On Windows, the situation is messier, which is why serious Python applications (e.g. Mercurial) bundle Python on Windows.
There's a lot of moaning about Python 2 vs. Python 3, but having worked professionally in Python and with Python applications for years, I have yet to experience problems worse than "oh, it'd be nice if Paramiko supported Python 3, so we could upgrade". (Which it does as of this March, so there's a nice little summer project at work.) Fact is, by now, the Python community is over the hump. All the important libraries support Python 3, and new applications are targeted at Python 3, with Python 2 compatibility being optional. (That's my professional experience, anyway.)