Ruby does lend itself to writing a lot of functional code. One can write Ruby so that it looks very similar to Bash or other Algol-derivatives, but it's also possible to write code which avoids side effects, mutable state, or variables in general.
Genuine immutability, ie. the deep copy variant, isn't that easy to work with in Ruby. At least not without a significant performance hit and who wants to add a performance hit on top of Ruby's baseline performance?
