It’s because most anvils were tuned to G. It was considered the original root scale step because anvils often worked best when tuned to G. But back in those days, it was called Sol (Solfège.) In french, Sol means ground, which is at the bottom. Ergo, every hammer driven by the smith was really just a sort of mating call that sent all the bottoms running to the smith’s doorstep. This is where the expression “she built like an anvil” comes from