To get to Python 3 support, you first have to drop support for Python 2 versions under 2.6, because 2.6 began introducing Python 3 compatibility features that make it possible to run the same code on 2.6/2.7 and on 3.x.

That's basically 90% of the time taken right there, since we had to do it one Python version at a time, giving people warnings that we'd be dropping support for 2.3, then 2.4, etc. so they'd be able to migrate up to a newer Python.