Simple. Nixon got tired of our creditors "holding our feet to the fire" when we stole from everybody. Nixon made it difficult/impossible for us to be held accountable for grand larceny.
So that we could borrow from the future to fund a war today. Same reason everyone else rejected the gold standard.
So we could print more money, A good read on the subject. "Paper Money" by Adam Smith, dell publishing Co copyright by George J W Goodman.1981
Unless we go back to carrying gold flakes in vials, and tweezers to extract them, paper is still going to be used.