what shortcomings does couchbase have, compared to mongodb? I see a massive gap of popularity between MongoDB and Couchbase.

What features\advantages does mongo have over couchbase?

I'm deploying DB instances in a cluster, between datacenters, where all servers accept incoming requests.
Is it not simpler to do this with couchbase, that has only one server role, and less configuration?
You are indeed correct!

It is very simple to deploy Couchbase instances and cluster them.  This is because Couchbase has a single node type which minimizes configuration, like you've mentioned.

What you might not know is that Couchbase has a built in caching layer, eliminating the need to install other caching services to sit next to your database.  This puts an emphasis on performance, which is critical in modern database.

UPDATE

I am a Developer Advocate at Couchbase, and listed a few advantages for you.