The troy ounce has been the standard weight measure for precious metals, including gold, for centuries. This is because the troy system is a more accurate and consistent system than other weight measurement systems, such as the avoirdupois system we use for everyday use.
In addition, the troy ounce is an internationally recognized weight measure, which means it is used the same all over the world. This makes it easier to trade and price gold internationally. This is also why the gold price is also expressed in troy ounces, this way there can be no ambiguity when converting the gold price from grams to troy ounces, for example.