I don't think it has much to do with diminishing supply of anything. It has everything to do with increasing demand for film. They've seen an opportunity and are taking advantage of it.
I also think the prices will come down eventually. We're going through a massive global deflation. What I'm worried about is these guys going out of business. I don't mind paying even a lot more as long as I know it'll be available. The Super XX Pan example comes to mind. It was the most expensive B&W film made, but I'd pay $100/box now if they still made it.