People only buy what they are exposed to. MTV only plays the videos provided to them by the record companies. Radio stations only play what the record companies push at them (it is a myth that radio payola no longer exists, record companies have just found creative ways to reward stations, like buying advertising or co-sponsering events, instead of directly paying cash to the DJs.)
The O' Brother soundtrack is proof that people will buy good music if they get a chance to hear it. No radio play, no music video (until after it was a huge hit.) But people came out of the theatre and went directly to the store to buy the soundtrack (in fact, that would be a good money maker for theatres, sell the soundtrack to films they are showing right inside the theatre).
File swapping is a golden opportunity -- it massively scales rapidly. The music biz's status quoism and greed blinds it to exploiting these golden opportunity without cutting off people's hands and hobbling techonology.