“Why did it take a greenfield project…?”

By definition greenfield projects literally means free from constraints.

So the answer is in your question: Why did it take a team unbound by constraints to try something new, as compared to a project with millions of existing stakeholders?

Single vision. Smaller team. What they landed on is a hit (no guarantee of that in advance!)

Conversely, with so many stakeholders, getting everyone to rally around a change (in advance) is hard.

In my experience this is about human nature/organisation and spans all types of organisations, not just python or open source etc.

It also looks like python would have got there, given the foundations put in place as noted in the article.