iTunes music hasn't been DRMed in about two and a half years now, I think. And honestly, things have moved quickly. When iTunes music was still DRMed, I think we were still in the process of transitioning away from CDs as a playback medium.
That said, I think the biggest reason iTunes has succeeded is because it's easier to buy the damn song than it is to go hunt it down. I suppose if someone emailed you the song file directly that'd be easier, but only marginally. When it's equally easy/easier to do “the right thing”, I think most people tends toward doing “the right thing”. The problem with DRM is it usually actually makes it harder to do “the right thing”. Oops.
That said, I think the biggest reason iTunes has succeeded is because it's easier to buy the damn song than it is to go hunt it down. I suppose if someone emailed you the song file directly that'd be easier, but only marginally. When it's equally easy/easier to do “the right thing”, I think most people tends toward doing “the right thing”. The problem with DRM is it usually actually makes it harder to do “the right thing”. Oops.