Was it just the time that the ecosystem needed to adapt to the Python 3 changes or were there essential changes in the 3.x versions up to 3.6 that made it easier to switch from Python 2?
Also, supporting 2.5 (or even 2.4) together with Python 3 was particularly inconvenient, so things sped up after people stopped caring about those (which took a long time because some people were running extended-support enterprise distributions).