Why is Ruby more suitable for Rails than Python?
I suppose we should not discuss the language features per se but rather the accents the respective communities make on the language features. For example, in Python, re-opening a class is perfectly possible but it is not common; in Ruby, however, re-opening a class is something of the daily practice. this allows for a quick and straightforward customization of the framework to the current requirement and renders Ruby more favorable for Rails-like frameworks than any other dynamic language.
Hence my answer: common use of re-opening classes.
Python and Ruby are usually considered to be close cousins (though with quite different historical baggage) with similar expressiveness and power. But some have argued that the immense success of the Rails framework really has a great deal to do with the language it is built on: Ruby itself. So why would Ruby be more suitable for such a framework than Python?