There are reasons, though they can be open to debate. One is that the value of our currency would be subject to world gold prices. There is a famous case of a ship bearing gold outbound from Sab Francisco that sank in a storm, and sent the dollar into wild gyrations as a result. Also, there is only so much gold in the world, and as our economy grows over the years the value of gold would balloon as a result. Like I said, though, the subject is definitely open to debate, and gets debated often.
Gah. *San* Francisco. Can't type tonight.
Ships have always sank. Certainly that would have seemed a very great thing at the time for at least several months, but I'd still opt for tying money to a metallic standard, which tends to at least bound politicians to the ground a bit, versus the unbridled constraints of being a mere currency among many? If that's debatable let's do it.