Economies of scale. There's a much greater market for auto rims than for Harley rims, so the costs of producing car rims is spread over many more units, lowering the price. Additionally, bike rims need proprietary threaded holes for the attachment of brakes and sprockets, while auto rims just need holes for the lug bolts to pass through, so there's less machining. Many aluminum car rims are cast, too, while the majority of custom motorcycle rims are CNC machined from billet.
If it ain't broke, don't fix it.