In the survey I took today there was no choice for Python under "Which technologies do you use...". I see 16,028 Python tagged questions on SO currently and only 6,556 Ruby tagged questions and Ruby is on the list, so why wouldn't they include Python when it's three times as important to the site's traffic, according to the audience?
We can certainly look at the activity per tag, but we're more interested in how this correlates with the technologies in the survey.
To expand on Æther's answer, there is little (if any) interest in Python from an advertising perspective. Plus, there don't seem to be too many commercial products available.
As for Ruby/Rails.. for whatever reason *cough*fad*cough*, it's pretty hot and there's a lot of interest from advertisers.
Perhaps SO's advertising clients don't care as much about Python as Ruby.
There is an "other" section though: