[csense]

Network effects.

When Python 3 started out, it had almost no libraries. So most projects at the top of the software stack couldn't use Python 3 because all the libraries they needed only worked with Python 2.

Below the top, to this day any individual library that wants to switch to 3 basically has to maintain separate forks for 2 and 3, because a lot of downstream users still use 2 because not all libraries are 3-compatible yet.

I think the Python developers are crazy for not using the proven __future__ import mechanism to allow new features to be introduced gradually and have new code interoperating with old code.