Comment Two words: (Score 1) 410
Back catalogue.
There are waaay more songs available through CDs than MP3s. For example, when I saw this article I went looking for the CD right in front of me: Chicago - The Musical (not the soundtrack). It ain't on iTunes, or on the Sony equivalent (which sucks, BTW; scripting bugs render it basically useless).
My point is that CDs will be driven by their sheer, established catalogue for a long time to come yet, as well as all the other reasons that have already been pointed out.