for a couple reasons... And I can think of right off the top of my head.
All wheel adatptors are NOT created equal - Heat treatment processes do vary among the manufacturing of the adatptors, which obviously affects strength and ductility
Spidertrax Spacers are not perfect, but they do have the FEWEST failures among available brands - so you are paying for a proven product that works, and as a result you are paying for the name as well. The adaptors aren't powdercoated, they are anondized - Unless you are talking about the price of the adaptors being used to offset the cost of other products they produce.