There’s a great discussion going on over at Hacker News about people’s opinions on what happened to CouchDB’s popularity as compared to MongoDB (and other NoSQL data stores).
My guess is that MongoDB took off lately as 10gen really gained traction with their outreach to developers while CouchDB is still fragmented despite the backing of Cloudant and Couchbase. I also found MongoDB easier to get started with than CouchDB. It is possible developers who recently learned about NoSQL considered MongoDB to be a better starting point to learn than CouchDB.
Also, here’s my introduction to MongoDB. Here’s my installation guide to CouchDB on Ubuntu (introduction to functionality coming soon).