I'm not sure they could have actually, I thought that was one of the stipulations of using Steam as a distributer.
There's tons of games that that use a different system inside of steam for DLC etc. For example Bioshock 2 uses GFWL for DLC, all the Trackmania games use its own crappy frontend for all addon stuff.
Yes, but that would require having a different system set up. As is they only need to set up a system for the initial release and then the one expansion, minimizing their need for an infrastructure.
I get that people have spotty internet connections, I spent years living in the sticks and Steam was usually a pain in the rear. But this is a small time developer looking for the most efficient system, and this is what they've chosen.