Using the Troy Scale to weigh precious metals is just a convention. I don’t know why it is done, but I would guess that using a separate scale shows the “chashivus” of these metals. Originally it was only for gold and silver but now we use it for platinum also (and maybe other metals as well).
Using the terms “ounces” and “pounds” is confusing, since they are not equal in the two systems. That is why we usually say “troy ounces”, much like Canadians (should) say “Royal litre”. Expensive metals are not normally thought of in large quantities, so you might have never heard anyone say “troy pound”.
The one unit of measure that is shared by the Troy Scale and the Avoirdupois Scale (I had to look that up, I couldn’t remember how it is spelled) is the grain. A grain of gold weighs the same as a grain of feathers. But there are 7000 grains in an Avoirdupois pound, and only 5760 in a Troy pound.