The answer is simple—Python changes because there is such a need. The competition does not sleep. Every few months a new language pops out out of nowhere claiming to solve problems of all its predecessors. Most projects like these lose developers' attention very quickly and their popularity is driven by a sudden hype.

Anyway, this is a sign of some bigger issue. People design new languages because they find the existing ones unsuitable for solving their problems in the best ways possible. It would be silly not to recognize such a need. Also, more and more wide spread usage of Python shows that it could, and should, be improved in many places.

Lots of improvements in Python are often driven by the needs of particular fields where it is used. The most significant one is web development, which necessitated improvements to deal with concurrency in Python.

Some changes are just caused by the age and maturity of the Python project. Throughout the years, it has collected some of the clutter in the form of de-organized and redundant standard library modules or some bad design decisions. First, the Python 3 release aimed to bring major clean-up and refreshment to the language, but time showed that this plan backfired a bit. For a long time, it was treated by many developers only like curiosity, but, hopefully, this is changing.