As someone who has been in mainly or partly-Ruby companies for the past 12 years - I suspect it’s because it’s different enough in its approach that someone who isn’t very senior will have a long learning period in Rails. Applications that are written in Rails are usually big monoliths and the framework is very easy to get started in, but difficult to be “extremely skilled” with (for example, it’s easy to get started in ActiveRecord, until you realize that it’s killing your performance if you don’t optimize the queries it generates, which forces you to break the abstraction wall pretty thoroughly, resulting in quite complex code). So I think a lot of “Rails” companies really want a Ruby/Rails expert to come in and actually help them sort out some of their complex Rails monolith problems, rather than someone they will have to teach Ruby and Rails.

And I know lots of languages and things have complexity and are hard to master but I feel like Rails experts especially are so uncommon to find that I feel like there are just always open reqs looking for them.