As I mentioned the last time this came up, I recently converted a medium-sized (about 30,000 lines) system from Python 2 to Python 3. The problem is not the language changes. Those are minor, and most of the new forms worked in Python 2.7.9. It's the state of the external modules. Important old Python 2 modules have been discontinued, and their replacements are not used enough or tested well enough to use reliably. I kept finding bugs that should have been found in the first months of heavy use of a module.[1] It took an extra month of work to find and work around all the external module bugs. These aren't obscure modules; they're ones most large server-side applications would need.

Porting was at least possible. Two years ago, I took a look at doing this, and there weren't enough modules ready to even attempt it. Now there are. There is progress. Slow progress.

The download statistics tell the story: "We can see that in the past year Python 3.x has grown from roughly 2% of the total downloads from PyPI to roughly 5-6%."[2] That's why the external modules don't work reliably on Python 3. 95% of the package use is on Python 2. Python 2 use is probably higher than that, since those are download statistics; PyPI doesn't see packages downloaded long ago and still running.

There's extensive denial about this among Python fans. That's a big part of the problem. Another problem is that PyPI is just a link farm - unlike CPAN, it doesn't centralize code hosting, bug tracking, support forums, and quality control. This allows Python fans to be unaware of how buggy the packages are.

[1] https://news.ycombinator.com/item?id=9378898, search for "quality control" [2] https://caremad.io/2015/04/a-year-of-pypi-downloads/