To cut a long story short, Active Record (especially at the time) wasn't super keen on generating super good SQL that would do as much as possible in database, and Ruby 1.9 arrived with stable ordered hashes. Then someone implemented basic ActiveRecord wrapper on top of key/value store (I think it started with Berkeley DB or similar), and from there NoSQL was off to the races as "new hip thing"[1]
[1] Meanwhile IBM IMS has been selling pretty well for last 40 years at the time ;)