It is the money. Pawn Stars gets a lot more viewers than some show about history. IRT is a big hit too.
Trading Spaces was the single highest rated show in the history of cable (up to that point) - guess what TLC became.
IIRC A&E said their highest rated show is Storage Wars - they even have theatrical trailers (aka commercial) for this season.
Not only are these shows getting ratings, they are cheap to make.