Owning gold was made illegal in the 1930's so that the government could inflate the currency and stimulate the economy out of the "Great Depression". You can inflate a "Federal Reserve Note". You can't inflate a $20 gold piece.

It was made legal, again, to own bullion and coin in the early '70's so that the wealthier among us could invest and protect their money from the runaway inflation that was going on.

Once things reach a critical point where too many U.S. citizens are investing in hard assets, such as gold, the Congress may be tempted to collect all the gold again.