The fact that requirements.txt files are by-default version pegged is an absolute nightmare for people upgrading. It simply does NOT work for a serious, large project that you'd like to keep up to date as dependencies improve. It might seem like it works great for a new, tiny, short-lived project, or to inexperienced devs.

The basic idea of pipenv, being able to resolve the dependency graph more cleanly and precisely, is far far far better - but there are still some usability improvements that leave something to be desired.