Why is the premium on Krugerrands a percentage instead of a fixed amount?

Because the price of gold fluctuates and keeping it at a percentage allows the price to increase or decrease based on demand. For example, if there was a $100 premium on Kugerrands and gold was $2,000 an ounce, such a premium might be reasonable, however, if gold somehow fell to $500 an ounce, such a premium would discourage anyone from buying Kugerrands. When the price of gold increases, more people generally buy gold up to a point, when the price of gold decreases a lot of people will start selling their gold rather than buying it.