Because banks and government wants something that they have control over. Where there are people who manages its every transactions and if something goes wrong they have someone to blame or accounted for. But with bitcoin it will never happened because of its decentralized nature.
In 2012, there were lots of internet talks "Buy 1 ounce Silver. End the Fed"
After all the readings here, Bitcoin got me interested. I will keep an eye on bitcoin whether it is accepted by worldwide people.
Gold and silver are universal so they can also be used as international currency if bitcoin becomes a single currency.