On both OSX and many Linux distros, Py 2 comes pre-installed but Py 3 does not. If I'm going to write simple scripts that I'm going to give out to people, it's nice not having to worry about them needing to fuck around to get the script to run.
Because python 3 was not the most stable at the time of distribution of that operating system. Why would I the developer of said operating system release anything but the most stable versions of the language? This would in turn make my operating system at times less stable.
She's ok with the occasional "sudo pip install ..." to get a library, but if my instructions started with "ok, first install Xcode and then install homebrew" the answer would probably be "it's ok I'll just do it by hand"
Its not that difficult.
Arch comes with Python 3 as default.