There's an iceberg of Ruby jobs that don't exist in the most obvious places to search, and because other languages have gained more spotlight (including anything that even smells of ECMAScript) there are fewer people searching for Rails jobs so the number of Rails jobs is pretty well matched to the number of job-seeking Rails devs. Also, early stage startups tend to hire from people they know, not from job search sites. Finally, there are many "full stack" jobs where work on the backend means Rails.

A simplistic metric like "job listings on angel.co" (especially if you're specifically looking for Rails in the job posting title) don't tell the whole story.