They said they worked with the company behind CouchDB and were not able to make it scale. So while you might accuse Canonical of not knowing what they're doing, I doubt you could say the same of the founders of CouchDB. Here is the official announcement :
> For the last three years we have worked with the company behind CouchDB
to make it scale in the particular ways we need it to scale in our
server environment. Our situation is rather unique, and we were unable
to resolve some of the issues we came across. We were thus unable to
make CouchDB scale up to the millions of users and databases we have in
our datacentres, and furthermore we were unable to make it scale down to
be a reasonable load on small client machines.