WordPress already has a facility built in for users and passwords. A plugin takes advantage of this by tapping into that user base. bbpress has its own database with users, passwords, and other data. To make the two installations seem like one, there’s something one has to do with bbpress to link the two user/password systems.
I also struggled with getting bbpress to match the templates of my WordPress site. Since SimplePress is a plugin, it exists completely within my current WordPress site and uses the same template system.