I guess not that many people stack other metals than Gold and Silver. Even the fairly expensive ones like Platinum, Palladium or Rhodium are not very popular among stackers. And those are way more expensive than Silver.
When you come to metals like copper, you really need substancial amounts to store a significant value, which takes a lot of room and is a serious pain if you have to move it, for example. Lets say a 10.000 dollar copper stack - how heavy would that be?
Of course there are a few lesser known, but more valuable metals. Like Tungsten for example or Chrome, Mercury, Vanadium ect. But they are still cheaper than Silver. And even Silver (at todays price) can be quiet bulky.
There is however the alternative of "Rare Earth" metals. Some of which are very expensive - even way more than Gold. But they are difficult to get, and there is a risk that a new technology could find a cheaper replacement for the stuff you own - and then its suddenly worthless.
Or you could buy 1 gram of Tritium gas for a couple of million dollars. Then you have a cool night light, at least. :)