As others are saying, lenses are not computers. The comparison would be to camera bodies, as they use CMOS sensors, processors, RAM, etc.... all things which are getting cheaper and cheaper to produce. Look up "Moore's Law" if you want more info. Anyway, I can't remember the exact details, but there have been times when Canon has introduced a newer body with better specs at a cheaper price than the model it's replacing. For example, this happened with the 350D/XT to 400D/XTi transition; Canon dropped the price $100 but the XTi was obviously an upgrade.
Really you just have to look at the competitive landscape. Apple is facing pricing pressure, especially on laptops. No a PC isn't a Mac (I'm a Mac user myself) but when you can buy 2 PC laptops for the price of the cheapest Mac laptop then things start to look awry. Same with Canon; when Nikon/Sony/Olympus/etc. puts out a new body with more megapixels at a lower price, Canon has to respond. There simply isn't as much competition in the lens world, since lenses aren't interchangable across DSLR lines. Thank goodness for the Sigmas and Tamrons of the world, or else we would be really screwed.