So many of the language features have changed in ruby programmer will have to edit every file in her project, and upgrade every single dependency, just to get her application running on a new ruby interpreter. Add that to the culture of "let's use as much 3rd party code as possible!" and the library writers emulating the core developers and changing their APIs all the time ... the process of upgrading an interpreter converges on "rewrite the entire application"
Which is what we do. And some percentage of those rewrites are in languages that don't have this problem.