Remember the old joke, "Python is the only language that has more web frameworks than keywords."?

What eventually happened was that the BDFL blessed Django and most of the others withered. Some had enough ecosystem, or were components of some other larger project, or had a niche advantage of Django, and they managed to survive.

I think the reason why web frameworks and data viz systems proliferate in Python is just that they are so easy to write, yet still challenging enough to be really fun, and you get a lot of highly, uh, visible feedback and reward for doing it.