I expect it's the same reason that .40ACP, .45ACP, .357Mag & others are so expensive. Lower production and (for the moment) higher demand. Why are .22's, .38's and 9MM's so cheap - high production & steady but moderately high demand.
It's all about what the market place will bear. Profit margins are built on high volume / steady demand / moderate price - or - high current popularity / low production / high price. The middle of the road rounds (.25ACP, .380ACP, .44Spl, etc.) remain high priced due to steady but fairly low demand & limited production.