Riak's sweet spot is with data sizes quite a lot larger than the typical RoR app.
Riak has a tunable consistency model, and there are a lot less applications that need tunable consistency compared with those that need the strengths of the SQL databases (everyone knows them, they just work) or MongoDB (designed from the ground up to be a developer friendly document database).
The closest Riak competitor is probably Cassandra (in terms of technical attributes), and you don't see huge numbers of Ruby apps using Cassandra either.
Riak has a tunable consistency model, and there are a lot less applications that need tunable consistency compared with those that need the strengths of the SQL databases (everyone knows them, they just work) or MongoDB (designed from the ground up to be a developer friendly document database).
The closest Riak competitor is probably Cassandra (in terms of technical attributes), and you don't see huge numbers of Ruby apps using Cassandra either.
http://wiki.basho.com/Riak-Comparisons.html is worth reading.