There aren't that many because there's not a big market for it. You ideally want the DB in close proximity (latency wise) to your web server or what ever uses it directly.
Why not host it your self? Deploying a server like MongoDB is trivial to get going.
For people deploying to cloud services, in-cloud latency is all the same no matter who the app/database provider is.
For instance, Heroku and MongoHQ are both using Amazon Web Services, so it wouldn't increase the latency to swap a MongoHQ database instance for a Heroku one.
Well, that's not entirely true. If you're running your own data source through EC2, you can make sure it's in the same availability zone (datacenter), which makes latency very small indeed.
Why not host it your self? Deploying a server like MongoDB is trivial to get going.