Perhaps one of the reasons that classical music makes up such a small amount of the sales is that people tend to hold onto and listen to the classical music they've purchased for a much greater duration of time. Add to this the easy access to good classical, nearly commercial free music on the radio (especially some of the npr stations). Think about it - I have cassettes of Beethoven and Mozart that I still listen to occasionally in the car (I have an older car without a cd player). Those cassettes are almost 20 years old. How many people do you know who purchased N'sync, Backstreet Boys, etc. CD's like they were going out of style, but haven't listened to them (or admitted to ever owning them) in years?
There's simply so much outstanding classical music that never goes out of style that it's easy to build up a decent collection and be able to listen to it over and over without excessive replays, and without getting bored of it. Not to mention, a lot of the classical music is cheaper than pop music. Bargains are everywhere. Also, the older classical classics provide a great variety of music within the genre of classical music itself. Thus, many of the fans of classical music really don't care a whole lot about modern classical music composers.
This is not to forget that if you mention "Bach, Beethoven, Mozart", most people have a clue of the styles of music you're referring to. A lot of time, people seeking some music from those composers will branch out into other composers from those eras. But, mention the names of nearly any contemporary classical composer (with perhaps the exception of Phillip Glass), and most people will say "who?" In the case of Phillip Glass, I myself would simply say, "I've heard of that guy."
Another consideration - given the price of CD's, even cheaper ones, who really wants to take a risk by purchasing something they've never heard before. And, while l listen to classical on the radio while driving in my car, the songs/symphonies/whatever last soooooo long, that it's rare that I have a chance to even learn who the composer was of a particular song, even when I would be interested in purchasing it. Then, since there's so much variety, I may never hear that song again. Compare that to pop music... switch the radio to a pop station and you'd have to be brain dead not to quickly learn who most of the major artists and new releases are.