As someone who has made the jump to 3 from 2, I wish 2 would die ASAP. It's a pain to maintain backwards compatibility with my various projects and 3 is demonstrably better.
I’m not sure of their reasons but Apple does not ship Python 3 by default on the Mac. If they did, I would switch. For me, it is mostly about being able to depend on default infrastructure and not introduce dependencies for other people; and generally, if Python 2.7 works it is better to let people use the default "python" instead of requiring something more.
What you're saying makes sense in some cases. In most of the use-cases I've encountered I want to run a separate version of Python than the operating system. This allows me to update Python independently to the OS. I was stuck supporting CentOS5, which ships with Python2.4, long after Python2.7 had shipped. Jumping straight to CentOS6 or 7 introduced quite a few variables (not to mention quite a few bugs in the standard library were patched and it was nice to take advantage of that without upgrading the OS).
