A combination of Unicode strings breaking a lot of libraries and Py3K not having a lot of user-visible improvements early in its lifetime. It built up a base of

A combination of Unicode strings breaking a lot of libraries and Py3K not having a lot of user-visible improvements early in its lifetime. It built up a base of people who said "they broke all my code and I don't see any reason to fix it". A lot of people recommended (legitimately) that you stick with Python 2 until Py3K was more widely supported. The momentum from this group persisted even after the point where it made sense to switch to 3. And so we are stuck, especially in enterprise where it could cost a lot of time and money to port because the transition wasn't managed well, both by users and the Python team.