Two reasons really, the first, and most potent, is that people are still buying them at the price they are at, they are still better than a lot of the competitors out there and of course, lower specced by present days standards, but people still buy them, I did, so they have no reason to drop the price.
The second reason 'price cuts' and 'clearance deals' lessen the luxury aspect of the company, not just apple, but any - it creates the illusion of more supply than demand therefore that is why you get a good deal, not because the company is good to you or are trying to give you the best deal, but because they have numerous units left to shift that they're making a loss on therefore they can cut the prices. If apple were to start lowering the prices between models, they would fall into this pit and they too would be casting themselves as a sales driven, rather than quality driven company (as far as the untrained consumer is concerned)
Obviously this is just speculation, but I know that some clients of my own in the past (luxury bathroom furnishings/luxury boat manufacturers) function on the same level, from a marketing standpoint, creating a luxurious, desirable item, means that those who want it will continue to want it, overall it's a very small percentage of their markets that wait for prices to 'drop' to get a bargain, therefore they don't allow for their products to be sold in discount warehouses and the like.
Quality, Performance, Function or otherwise Apples products are still 'luxury' items, very few people out of those who own them actually *need* them, I know I for one *need* my iMac for work, but do I really need my iPad instead of another tablet? No, do I really need my MBP instead of a netbook/ultrabook? No, do I really need my iPhone over a phone that has a useable battery life? No.
But, I own them, I'll probably continue to own/purchase them, and the price isn't what drives me at all, the quality of the products is enough to make me crave them anyway.
TL;DR - Price indicative of quality, Reduction in cost cheapens appearance of company in consumer markets