Two Reasons

1) Syncing issues -

I have two computers that share music. The music is stored on my Apple Time Capsule external hard drive and is accessed from there. But, when I buy music on one computer it doesn't automatically show up in iTunes on the other computer. I want to be able to buy music from both computers and have them both constantly in sync. After much research I've found that this is not possible except in a very tedious method. So, I don't really want to listen to music in iTunes or buy more music. It's too frustrating.

2) It's too expensive - 

I'm sorry, but I want to buy A LOT of music and when I think about how much that will cost it is daunting. So, I end up buying very little. Now I only buy music that I think will directly influence my singing style, music that I can consider directly useful to my job.

I believe these issues will be resolved in the future, but for now I'm stuck in the middle.