The biggest problem is fundamental to Python and a lot of other scripting languages: no static typing. Running Python 2 code direct on Python 3? All looks good, until _somewhere_ you use a method of str that isn't on bytes, or vice versa, or something that was a list before is now an iterable and it blows up in your face... but _only_ if code execution reaches that line. So you have to test _everything_. And if you're a library/module, that means testing with _every user_.
Secondly, it seems to me the Python team didn't want to make it possible to write code that works correctly on both Python 2 and 3 simultaneously. They wanted you to write code that only works on Python 3 and tell every single Python 2 user to get lost. Essentially, they wanted to co-opt you as their agent of change; burn your own reputation by abandoning your userbase (or worse, start proselytising Py3 at them) or double your own workload to maintain two codebases. Who wants that?
It's fundamentally an attitude problem. A desire for there to be only one right way to do things, a change of mind of what the right way is, and kicking the chair out from anyone doing it the old right way; the only compromise being to kicking the chair very very slowly. In Java, you can still run code that uses Vector and Hashtable from 1996, they're not even deprecated. They're used by _very_ little code since generic List and Map APIs superceded them 2004, but they're still there.