I would imagine Python's approach to introducing new language features had a lot to do with it. Having to go through the PEP system takes some time, and changes like these tend to be reserved for minor-version releases. All in all, I love the PEP system, it's such an open concept and I've been surprised by the amount of quality proposals that get implemented. Wish Go had something like it.

The change to UTF-8 source encoding also changed the legal set of characters for identifiers, and specified how to normalize them. Which in turn is the reason behind this thing I posted on Twitter a while back:

https://gist.github.com/ubernostrum/b7b705bf21b86a1b5c1e2c9f...

And also is a big enough change to not really be something that could happen in Python 2.