Back when countries all existed on the gold standard, governments needed to be able to back their currency with huge volumes of gold in reserves. In order to trust that a government's money was worth anything they needed to have sufficient gold in reserves they could potentially pay it out if money holders wanted gold instead. However, most governments began transitioning off the gold standard around the 1970s meaning that they had no need to hold enormous gold in reserves.

So why is the United States still holding so much gold when Canada is not? It costs a ridiculous amount of money to hold onto gold. You need a facility to keep it in, you need to guard it and the gold doesn't really do anything for you while you're holding it. Canada has realized that instead of paying all this money to hold, they can sell it and buy bonds, assets that don't cost any money to keep and actually make you money. The United States on the other hand keeps most of its reserves in Fort Knox, a military facility that they would still be paying for with or without gold because it guards numerous other secrets.