Because there's been not enough carrot and too much stick.
The only real killer feature of Python3 is the async programming model. Unfortunately, the standard library version is numbingly complex. (Curio is far easier to follow, but doesn't appear to have a future.)
On the down side, switching to Unicode strings is a major hurdle. It mostly "just works", but when it doesn't, it can be difficult to see what's going on. Probably most programmers don't really understand all of the ins and outs. And on top of that, you get weird bugs like this one, which apparently is simply never going to be fixed.
I wonder how many people feel that migrating to Python3 would have been worth doing in the absence of being forced to do so.
Dropbox invested three years of work, actually hired Python's creator, and are still not done. What are they getting out of it that they wouldn't have gotten if Python2 simply had been maintained?
This is backwards thinking.
Yes, it's expensive to upgrade from Python 2 to Python 3, but it's also expensive for the Python project to maintain 2 versions of Python indefinitely. If someone wanted other than the core Python team wants to step up and maintain Python 2, they are free to do so, it's open source. But failing that, expecting the Python team to support the older/ less functional version of the code indefinitely is unrealistic. Corporate owned languages have even shorter lifecycles for exactly this reason.
Only if they name it something completely different from python or py-anything. Guido refuses to allow anyone to just step in to maintain py2.
Tauthon is a project that aims to keep compatability with py2 while adding whatever features of py3 that won't break py2 and to have a maintained py2.