As a non-Python pro, I cannot say why one version over another. However, arguing that the old one is bad just because it is inconvenient isn't valid.
As someone who struggles with versions of Python on my Mac and on production servers (and with code that runs on 2.7 and 3.6+), as far as I'm concerned (as a pragmatic solutions-first person goes), I cannot discern the difference between ancient Python and new Python.
There are really few languages with such impact as Python. So we cannot blame the Python community for this situation as they probably really labored over their decisions regarding compatibility and versions. But in retrospect, I would have preferred they killed off the old version long ago. It would definitely have made life better for the users in the long run.
In this case I'm included. Python is not my first, second, or third love. But it is the most available and the simplest tool available to glue things together. CSVs, json, web apis (private and commercial), etc., are all so easy to do with Python.
So my guess is that there are a lot of users who may not even realize the benefits that Python3.x gives vs 2. "We" don't know or care about features we don't need. But we do feel the pain of modules that only work for one version.
In hindsight, I would have voted for a hard break from 2->3 perhaps 2 years ago. I suspect the ultimate human time effort would have been less than we waste now straddling or stumbling with two versions.
In comparison PHP and Java always successfully transitioned to new versions, keeping it backward-compatible and do baby steps instead of a big incompatible cut. PHP canceled the ill fated PHP6 fork, and went from PHP 5.2 then up to 5.6 and then jumped to PHP7 (as several PHP6 books got published about an alpha version).
Language with a rocky transition (mostly due to incompatible syntax) were C#/dotNet 1->2, Perl 5 -> 6, Lua 5.1 -> 5.3, Ruby 1 -> 2, Swift 1 -> 2 -> 3, Rust 0.x -> 1, and more
You can run PHP3, PHP4 and PHP5 projects with little or no change at all, code dating back to 1990s with PHP7. If you cared a bit and adjusted your code over the years, most changes are announced many versions ago and got deprecated. E.g the original MySQL API had been deprecated for a decade or so years, and only got removed with v7, yet it's easy to update the code to the newer APIs, as it was possible since early 2000s, when the newer API got introduced and stayed unchanged since then. And you could use a shim too.
PHP and Java (and several other languages) have really kept an eye on backwards compatibility, you cannot deny that or paint it in another light.
It's an enormous amount of effort. It's been a decade and the end is nowhere in site. Python 3 is a tragedy.