Because the copper will flatten out. I know way back in the day everyone used copper orings. When I started using them in 1984 we used stainless.
Also, to do it the correct way, you really need to oring the head and the block and put the wire in the head and use the block groove as a receiver groove. It will seal about anything you can throw at it and you are less likely to have oil and water leaks from the head gasket.