While I can't say for sure, one or the reasons I seem to recall from the Python 3 transition was that the Python 2 design was pretty much a dead end. There where so many limitation and wrong design choices that it would keep the language from moving forward. That does seem a little aggressive, but it does feel like Python picked up a lot of steam once Python 3 was viable (something that happened way earlier than many care to admit).
Our code base wasn't huge at the time, a few 100.000 lines of code. Getting everything running was a week of work for three people. Sure many had way more complicated code, and some depended on some idiosyncrasies of Python 2 making thing more complex, but a lot of people acted like the Python developers shoot their dog. Mostly these people either simply didn't want to do the work or their particular niche was made ever so slightly more complex... or they depended on unmaintained libraries which is bad in it's own way. Python 3 was always going to be able to do everything Python 2 could, yet a large groups of people acted as if that wasn't the case.
Still not the best transition ever devised, we had to wait until 3.2 to get the speed to a point where it's wasn't an issue for all but the largest users.
Our code base wasn't huge at the time, a few 100.000 lines of code. Getting everything running was a week of work for three people. Sure many had way more complicated code, and some depended on some idiosyncrasies of Python 2 making thing more complex, but a lot of people acted like the Python developers shoot their dog. Mostly these people either simply didn't want to do the work or their particular niche was made ever so slightly more complex... or they depended on unmaintained libraries which is bad in it's own way. Python 3 was always going to be able to do everything Python 2 could, yet a large groups of people acted as if that wasn't the case.
Still not the best transition ever devised, we had to wait until 3.2 to get the speed to a point where it's wasn't an issue for all but the largest users.