None of the posts in this thread really answer the question to "why python over ruby", they just talk about all of the nice features of current python development. The thing is modern Ruby has all of those tool sets as well.
Before you even being to make the statement "Why are more Startups using Python over Ruby?" you need some data. I'd be willing to bet that more places do in fact use python over ruby, but that number is a lot closer to 50/50 than you think. In fact, it is probably so close that this argument becomes pretty silly.
I once heard a successful startup founder describe the choice to use Python over Ruby because a couple of years ago getting started with Ruby was too difficult for them. They had spent a day trying to setup Ruby, Rails, etc and nothing was going correctly. After that they tried to install Python, it worked, and from that point on code was written in python. This story isn't elegant, sexy, or thought provoking, but it goes to show you that there are all sorts of crazy, silly, and stupid reasons why some people pick one language over another.
Lets talk about reasons why startups are _NOT_ using Python over Ruby.
Python is not taught at more schools than Ruby. Well, I couldn't find a list of schools teaching Ruby, but search for the Python list (hint, its up on python.org). There are only a handful in the US and these look like nothing when you consider all of the places that use Java.
Python's syntax is not easier or better than Ruby's. Nor is Ruby syntax better than Python syntax. Anyone trying to compare the two is just looking for a flamewar. Omg significant white space vs lol meta programming type arguments.
Both languages have excellent documentation and community. Very hard to compare these sort of things.
Django is a great framework, but so is Rails. Both have very vibrant ecosystems that are just exploding right now.
Tools - great tool sets in both. If a Ruby tool comes out that is good you can bet your ass there is going to be a python port of it in the next few years (and vise versa of course). A single great tool is never bound to one language/framework.
Implementations - both languages have a ton of cool projects that revolve around finding alternative ways to rebuild the VM.
Go ask people why they chose to write in a language/framework. They are going to give you answers like "it's what we chose", "i dunno, we just knew it". Comparing things like documentation, community, MVC Frameworks, tools, syntax, etc with two languages like Python and Ruby is pretty silly because its going to become a wash every time.