If we're comparing CouchDB's popularity to that of MongoDB's, it's simple - 10gen. 10gen has done a very good job generating lots of hype around their product. I believe they have people dedicated to this task (aka, evangelists). CouchBase on the other hand, has not done so well at this.
And that hype turned me off. I have an allergic reaction to it. Especially when they were caught acknowledging they didn't have default durability for their writes. Then suddenly they removed their benchmarks from their site and made some comment about how data will be lost anyway, so it is not a big deal.
Not moot for someone who already chose a different product because of that. These decision are extrapolated and people infer the quality and seriousness of engineering from such decisions. Those, in my opinion, matter more than marketing or flashy websites.
It's also that CouchDB is (or at least used to be) rather slow compared to MongoDB.
10gen sure has done a lot to keep MongoDB growing despite many people having their homework (or essential customer data, or...) eaten by MongoDB; so part of it probably is attributable to better publicity.
There is a reason they are fast. They didn't use to have durability. Think about that -- it is a database product that until the last version didn't have durability and its writes were not acknowledged with a response. If you store you data in memory and write it to disk sometimes, you can write very fast.