I think it's just that Ruby enjoyed a brief surge of hype which pushed it to the forefront of the startup founders' mind; this has now ebbed a bit and Python use rises up to its natural level.
I wonder if Ruby will come back a bit, though. There hasn't been much hype, but in the last few months the Ruby community has really gone for broke, with rails 3, ruby 1.9.2, rvm reaching maturity and bundler to handle dependencies. Things are changing so fast I think the magnitude of the changes haven't sunk in for most people but it's a whole new environment and really impressive IMO.
At some time I decided to give it a try and wasn't disappointed.