"...Sterling silver is not in demand as pure .999 silver."

Sterling silver is .925 fine. The jewelry industry uses lots of sterling silver. .999 fine (pure)silver is not used for jewelry that often because of it's relative softness (and expense). .999 fine silver is often used in electronic contacts.

Silver that was used in our coinage prior to 1964 was .900 fine (or 90% silver). I'm surprised that IGT has not switched over to .900 fine silver centers in silver strikes to better market their $10 product to casinos.

One of the biggest users of silver used to be the photography industry, but with the avent of digital cameras and such, that use has probably been diminished significantly.

The cost of mining silver also has driven up the price. It does not pay to take silver out of the ground below a certain price level. Increasing fuel and refining costs make mining operations more expensive, hence the price of the final product (silver) goes up.