Edit: I've been wanting something like this for ages. Sometimes, I'd draw graphs on paper (similar to what Sourcetrail is doing) to make sense of code. Even my code :).
Ruby is so dynamic that code navigation is not easy. To resolve a method which is implemented via method_missing() in one of base classes, with respond_to?( ) in another base class, you need to instantiate the class hierarchy first and pretend-run some class methods.
Navigation in a statically-typed codebase is much much easier to implement.
Python's been implemented and works well enough with sane codebases. I imagine given Ruby's strong use of conventions, common usages could be covered easily given time.
Edit: I've been wanting something like this for ages. Sometimes, I'd draw graphs on paper (similar to what Sourcetrail is doing) to make sense of code. Even my code :).