The story I've always heard is that when indoor plumbing was first introduced, in most cases this would have been back in the 1800s, there was only cold water. Since most people -- about 90% -- are right-handed, it was logical to put the pump handle or faucet or whatever other device on the right side. Later, when homes started getting hot water, it was natural to put that faucet on the other, or left, side.