Because Badget/Bolt are key/value stores and MongoDB is a document store.

I can't imagine how anyone can model even moderately complex use cases on a key/value store. Even with some sort of ORM filtering and manipulation would be very cumbersome.

Mongo serves as a data store for job metadata, which is in turn then access to be displayed on the dashboard.

We are currently working on having the database component as something more pluggable, so down the line you will be able to use the DB of your preference :D