There is no legitimate reason for the lack of EF-S L lenses. It is purely a marketing decision.
In the old days of film, people had no hesitation putting L lenses on their lower end $300-$500 camera bodies. Canon encouraged this. Look through some old photo magazines. Canon generally promoted their wide range of lenses more than individual bodies. In the old days, when people changed cameras less frequently, the money was in lens sales.
It's a bit different today. A recent stat published on CR suggested that most people stick with their kit lens and don't invest in a second lens. However, people are clearly updating cameras a lot more frequently. Canon's advertising is predominantly aimed at selling new bodies.
How do they make more money? By selling more expensive, higher margin bodies.
And how do they do that? By convincing people that only the plebs would use a crop body. After all, if you were serious about image quality, you would be using full frame. Plus, you can't be considered a serious pro unless you're using a full frame camera. If you use a full frame camera, your pictures will exude pure awesomeness. And if you use a full frame camera, you can use the L lenses at their designed-for field of view.
Of course, it is not just Canon promoting this view, but also "the internet' in general.
Now, maybe Canon is right and they are doing us a favour encouraging us to move to full frame. Full frame cameras probably are (for most applications) "better". I know that I'm keen to see what the 5Diii will look like. Not only am I interested in technically better image results, but I want to go back to using my lenses for the purposes they were designed. I like using well built lenses. My 10-22 works as well as my 17-40, but the 17-40 is a joy to use, works very smoothly and doesn't have a focus ring that wobbles. I used my 70-200 a lot more when 70mm meant 70mm.
Anyway, back on topic. Canon should produce EF-S L lenses. There is no reason for them not to. In fact, if I was them, I'd take advantage of the crop sensor and produce a range of fast, small, L primes. In particular, a 12, 24, 50, 85mm.
The other argument that people use against EF-S L lenses is that L lenses should be compatible with the higher end professional full frame bodies. What nonsense! Although I don't have access to Canon's sales statistics, I'd be willing to bet that there are more amateurs out there with L lenses on crop bodies than pros with 1 series bodies. Plus, I find it hard to believe that a 1Ds user would mistakenly buy an EF-S lens purely because it had an L designation. And if they did, well.....
(Sorry for my "rant for the day" but I get upset when people infer that I'm not worthy of using a particular product, simply because I don't use the right camera body.)