The Python 2 to 3 transition.
The process of going from Python 2 to 3 have taken forever. It seems like Python 3 finally is the standard now. But it was not until the latest version of Django where Python 2 was deprecated. There are many different reasons for it. But it has taken way too long.
Part of the reason it has taken such a long time was that Python 2 continued to be supported alongside with Python 3, and the end of life for Python 2.7 will be in 2020. My view is that this have taken much longer than needed because of the Python 2.7 version continued to be a up to date version of Python for as long as I did. This have meant that developers of libraries and frameworks postponed adding support, and users postponed starting to use it because none of their libraries supported it.
I get why they continued to support the 2.x branch, but I think it would have been a much faster transition if it was a little bit more forced.