Unlimited ability of the gov't to create money to fuel vote buying schemes and guaranteed profits for the banking cartel.
I always thought that it was because of the volatility of gold prices caused in times of crisis by emotional traders in comodities.
Gold by itself is not so useful as a metal. It is pretty, malleable, good conductor but its scarcity renders it uneconomic for most uses. There are better alternatives.
Gold is an 'emotional' commodity. When the oil crisis hit the USA in the 1970s, Gold went through the roof in value. If the dollar had been pegged to gold, the volatility alone would have brought on collapse in financial markets.