You may not find some material on itunes (that even had been there). Some artists take issue with their material being on there through their labels. Most recently Pink Floyd took EMI to task for offering single track downloads instead of requiring full albums.
I did buy a lot of stuff on CD in the late 1980s, but I remember that back then some of the stuff was never put out on CD. Modern English’s “After the Snow” and “Ricochet Days” were both incredible albums (forget “Melt with You”, that got played way too much) and I’ve never been able to find either of them.
The only stuff I buy from iTunes is stuff that is impossible to buy on CD.