Well, God works in mysterious ways and He speaks to certain individuals and certain peoples at certain times with directives specifically for them and none other. I read an article a while ago that a rabbi did a study of the various fibers and ornaments prescribed for the cloth for the priestly class and discovered that the clothing acts as a shield against negative ion buildup. Being a people wandering out in the desert at that time, apparently the constantly shifting sand would cause some kind of static build up that would "cling" to you and over time might weaken the immune system. The fibers and such used in the clothing were such that it helped to discharge this ionic buildup and therefore not stress the immune system as much. If you "adulterated" the types of fibers, you lost the protective qualities of the clothing. But nomadic-just-freed-slaves wouldn't understand the first thing about ionic charges from shifting sands and weakened immune systems from those charges and whatnot (they could barely even cook or feed themselves since for generations they simply lined up at the feeding troughs after the days slave work), so the easiest thing is to just say "Hey! Do it this way or else!".
But who know, ya know? Seems arbitrary to us, but God knows what is best for who at what time, and it may be different for someone else at a different time. God is colorful that way, eh?