Why are class B's & conversion vans so expensive?

I think as a single person, I would have chosen a "B." Hubby and I camped in a van in our younger days and loved it. I could probably fulltime in a B by myself, but as a married couple planning on fulltiming, we need more space. It's not just the bodies, it's the hobbies. Fishing, quilting, camping videos, etc, all eat up space.