1) People listen to AM/FM radio instead
2) They prefer to own the physical CD
3) They're spending less on entertainment
4) They don't listen to music on their computer
5) They're satisfied with their collection
6) They don't spend as much time listening to music
7) They don't own a portable digital music player
8) They don't feel comfortable using their credit card online
9) They don't think that downloads are a good value, their afraid of spyware/viruses, downloads are too expensive, or they have no time to learn about new music (that seems like 4 reasons to me).
All that being said, Crupnick also noted that among the 13-25 set, price and access to shared music files rank higher than the rest of the population.
There's not much mystery here except for the order (number 2 seems too high to me), but now there seems to be some quantifiable evidence to what we all thought.
Check out my Big Picture blog for discussion on common music, engineering and production tips and tricks.