Not sure this is entirely true. My last two purchases from iTMS were Europe and Megadeth, two bands who were big in the 80s. For iTMS to sell something it must at least be worth acquiring the rights, a digital copy, some artwork and a little blurb about it. With a modern recording, all those assets already exist, I'd be surprised if iTMS and major record labels don't have a private API to bulk transfer new releases into it. For older recordings, a human would need to do it manually, and humans are expensive (esp. if one or more of them are lawyers). It's more involved for iTMS to do that than it is for a conventional store to just have a physical CD in its warehouse.
Interesting points. I expect iTunes is not to fault here at all. The fault would lie with record labels being too lazy to look at the back catalogue (beyond the million-sellers).