This seems to happen with many successful horror franchises, i.e. Halloween, Friday the 13th, Nightmare on Elm St. Few horror films make it big, so when they do they milk it for every penny they can get. The horror movie, particularly the gory slasher flick, works particularly well for sequel milling as the makers realize their target audience doesn't ask for story or anything, just people getting killed with the franchise's telltale signature, so other than that they can make up any shit they want and get away with it. I never cared for Saw anyway, so I don't care.