The Literal Commandment
If you make an altar of stone for Me, you shall not build it of cut stones, for if you wield your tool on it, you will profane it. (Exodus 20:25)
Profane literally means "marked by contempt or irreverence for what is sacred".1 If the Israelites make an altar of cut stones (i.e. using metal) then they will make the stones (and thus the altar) a common thing. Common things are not treated with the reverence that G-d demands in our activities involving Him.