When you have +100,000 LOC written over many years by people with varying degrees of experience and familiarity with the language, Ruby/Rails becomes unmanageably difficult a lot faster than other languages. It isn't the only one though.

Not that you can't have a great, large Rails codebase but the "guardrails" are a lot less present than in some languages and it can be harder to try to clean up the mess once it's happened.