Think price!
Rock Auto has the same pricing as my local auto parts stores (my local stores would have to order a set of 8) @ just over $60 each, then add shipping at RA. I can buy new upgrades replacement sets of 8 for $350 a set, with free shipping, and remanufactured in the $200- $260 range (with a "lifetime" warrantee, what ever that is suppose to cover), with free shipping. I don't mind giving my local stores a bit extra, its nice having them here, but double the price is a bit excessive.