I think the reason why Python never went all in on a single web framework is just because the Python community itself is much larger and more diverse than the Ruby community. Consequently there were alot more opinions and alot more people willing to develop those opinions into separate projects.
Maybe you're right that there are advantages to converging on a single framework, but I'm not so sure. Rails is omakase, so if there's some part of it you don't like its not really easy or a good idea to change it, and you don't have any other options in Ruby. Django is also omakase but if there's some part of Django that you really disagree with you can just move to web.py or flask or pyramid. these are all built on the common core of Python's wsgi module so these are just different opinions on how to accomplish the same task.