Primarily it's because of libraries. Python does not even have a solid standard library, there are at least four very solid 3rd party libraries I would love to use and have used some of it already:
- PyQt4 - I'm a big Qt fan and used it in some C++ projects. PyQt4 is the best Qt binding to a dynamic language.
- Pygame - a solid binding for SDL
- Twisted - I didn't use Twisted until now, but read many positive things about it and have to check it out.
- Django - a very solid web framework
Yes, Ruby also has some interesting projects, but I think what sells me most on Python is PyQt4... the Qt4 bindings for Ruby are not that great and lack documentation.