I mentioned this in another comment below, but scientific packages were often ignored when building web services and APIs and that's the context in which Django/Flask were built.

At the time the scientific packages were extremely heavy and best avoided in production environments. Some people did it anyway but removed it once they needed to scale. Simply installing these packages was a huge pain and blocked one's ability to provision new machines, for example.

PyPI was indeed a big benefit. Ruby Gems are also what made Ruby so competitive at the time.