First, it's like all scuba gear in general which has a high markup due to it being sold in relatively low volumes typically through small resellers that have to carry stock that may or may not turn quickly. Having shelves full of merchandise that doesn't sell quickly is a lot of money to tie up in a store, so you need to get a good rate of return on such items. Stores like Best Buy can sell consumer items at a small markup due to turning products quickly off the shelves.
Then more specific to underwater housings and gear, this is a much smaller market than the scuba market itself since many divers don't buy this type of gear. So it wouldn't surprise me if some of the higher end housings aren't built to order, and they may only sell very small numbers of them. Cameras sell like consumer items, in fairly high volume for the lower end cameras, and decent volumes at the higher end cameras, since many more photographers do non-underwater work. Then factor in that camera models that change all the time so they have to continually re-tool the housings for the next camera. Add it all up and you are looking at a product that will cost significantly more than a typical high volume consumer product. My guess is that without modern computer based design and layout, CNC, and low cost plastic molding that housings would even be significantly higher than they are now.