Because the Cambrian doesn't show even one single species that is alive today. If the Cambrian "explosion" indicates a creation event, it is a dfferent creation event than described in Genesis.
That is why most Christians before Darwin were Old Earth Creationists. They felt that the geologic record showed multiple creation events over a very long time. Young Earth Creationism really didn't get started until after Darwin published his theories -- in fact, I think that YEC was a sudden
reaction against the proposal that humans evolved from "lower" animals.