Perhaps the real issue here is that Linux's excellent package management make it significantly harder to installed a 'second' version of Python, or in fact any package. You either need to compile from source, or install a binary under a tree that doesn't affect the core package, or do a chroot-install, or something else that's equally complex.
Windows often makes this a lot simpler, although sometimes the registry enforces single versions. At least without dependency management it's easier to install the same version twice using a simple installer.
While package management makes things simpler, if there was an easier way to install two versions of Python and choose which applications use which version, you could make a gradual transition to Python 3 without breaking core distro features.