I can only offer a personal perspective on the reasons the transition to Python 3 isn't happening quickly, but I think it's salient.
For work, I maintain a number of python scripts & modules, which were written when Python 2.4 was current. These scripts work on Python 2.5 and 2.6. They are not compatible with Python 3.0, and any changes to do so won't be backwards compatible with Python 2.x, so to support Python 3.0 I'll need to fork my code.
So, here's the obvious question. Why should I move? It's a significant amount of work to do it, requires that I maintain two different codebases and I get no benefit at all from updating to Python 3.0.