i think a lot of the reasons I've seen come down to business reasons, not technical. Someone wants an app fast, like now, and MongoDB is fast to setup and get running with.
Usually first thing you need to do is write a REST layer on top of it, and with CouchDB that part is just done already.
Obviously there's certain kinds of data I wouldn't put in Couch, or any kind of NoSQL database.
You need to know what the right tool for the job is,but I just want to figure out when that tool is mongo.
It's only popular because there's less of a conceptual gap between mongo and the relational database tools that a lot of people are used to.
Couchdb on the other hand requires you to actually learn and use map/reduce.. which is a pain for people who don't feel like having to learn something new, but Couchdb is MUCH MUCH better in a lot of ways and Mongo is pretty much fundamentally flawed in my opinion
I do wish rackspace luck though with their offering. I think it was smart of them to create this mongodb product for one simple reason: a good number of people are already using mongodb so it makes sense to help them get the most out of it.