Because Shimano has starved the channel of 9-speed parts. Scarcity drives up price.
That's really not fair as it makes it seem like Shimano intentionally withheld product to drive up prices.
In the last year or two of 9-speed, before Ultegra and 105 went to 10-speed, Shimano experienced an unexpected great increase in demand for 9-speed components, mostly in Europe. They got caught in a stock-out situation everywhere. You couldn't get 9-speed cassettes for a while and the popular 12x27 cassette was backordered for months. They would rather have had the product available but got caught by demand that wasn't anticipated.