I think it's largely because of Ruby's deep association with Rails. This is nothing against Rails, per se, but so many people only think of Ruby in association with it that they just never reach for it in other contexts. Personally, I think Ruby is a great utility scripting language, really good for the sort of small projects that many people would probably have used Perl for two decades ago.

(Having said that, I suspect that the Ruby community's love of "first, install a Ruby version manager" does it no favors when it comes to adoption for these sorts of small projects, although the same is arguably true of Python.)