Riak is horribly unfriendly as a database: no SQL, it exposes eventual consistency directly to the developer, it’s relatively slow, and Erlang is a fairly unusual language.
While you can run Riak on a single server, you’d have to really want to.
Its strength is the ability to scale massively, but not many projects need that scale, and by the time you do, you’re probably already using some friendlier database and you’d rather make that one work.
Though it had a couple years head start when there really no other options for people wanting that kind of kit.
(I can't, of course, speak to the truth of this, only that over a couple decades of knowing the dude in question and working with him on and off he had sufficient Clue that I expect he did put in the effort before coming to that conclusion)