tl;dr - Python had no web frameworks built in or community favorite. Rails was a complete re-think of how to do web apps in the age of XML (acaffolding, convention over config, etc.) Every language since has now realized this and copied Rails. This is a good thing.
if you want a story while drinking coffee, read below...
I was a serious Python guy from 1996 til 2006. I still like and respect Python. Before Python, I was a wanna-be Smalltalk fan. (This is important towards the end)
Python always had a philosophy of 'batteries included'. This means that it will have libraries for just about everything you need as supported parts of the language. Contrast this with Perl where CPAN quality was very hit or miss. Then contrast this with Lua or Scheme. They had too many libraries and none were standard. Most of those were, again, not supported or not of production quality.
As a result of 'batteries included', Python was just a dream to work with. Everything was doable or easy and performant 'enough'. I could go on, but that still wouldn't do it justice. They had a great language design, great libraries, and portability. Python is still a great framework. They didn't have a good 'CPAN'. This was ok, since the built-ins were good. Outside of that, it was a bit of a mess.
However, over the years, it became obvious that Python had a huge flaw. They did not deal with HTTP or HTML well. People don't even talk about this anymore, but the promoted framework was 'Plone'. This framework was so complex and convoluted, that building a simple web app was just not possible. It is not spoken about anymore. New frameworks popped up as a result, and very few of those were much good. This was a huge problem. Most people had to build their own production system. I had to switch languages for a job in 2002, and I didn't deeply follow the web framework discussion in Python for quite some time. I can tell you this, though. There wasn't a single framework that popped up as the winner or as exciting.
I'm typically a conservative. When I see a new thing, my typical take away is "this has been done before, what did they not learn from prior art". For example, lets say someone creates a new OS. If they don't really understand Unix, they will get it wrong.
When the first Rails screen-cast came out, I took a look. There was a lot of buzz that was increasing. I go, 'what the heck, it is only 5 minutes and it is a video.'
What I saw was an excellent architecture for building web apps. This architecture was better than anything else out there by quite a wide margin. They supported modern HTML. Convention over configuration. ActiveRecord was truly a treasure. The default site actually looked relatively decent (vs. the typical programmer, zero whitespace, huge serif, Netscape circa 1998 site). They URL conventions looked good. This architecture was human friendly. These people understood what a modern app should look like.
I was sold. Just from that screencast.
I believe in using whatever language is needed, if the framework is excellent. I wanted to build excellent web apps, so I learned Ruby. Ruby was easy to learn, and it is as good as Python. Much to my surprise it was the first language to pull off having Smalltalk blocks! People didn't even notice this and now they really like it. Everytime I showed this to coworkers, they would go 'what is that? I don't understand'. When people eventually learn Rails, they would go.. "check out these block things. They
So, in summary, Python didn't have a web framework. Rails was an excellent design, compared to any languages framework. Rails is pretty much going to be here for quite some time.
BTW, at this point. Rails is not my first choice. I think it is too heavy and web architecture has shifted a lot. Still, it would be in my top 3 choices.