It fell behind in terms of perception while Mongo was chasing benchmarks early on but has seemingly continued to just keep chugging along with it's design decisions.
General community, "You sure about that? I mean...benchmarks."
Couch, "Yep...we're sure."
* few years later *
General community, "Have you seen CouchDB? This thing is really solid. Did you ever read that Hacker News thread about how terribly unreliable Mongo is?"