Because there's too many damn baby boomers!
I don't think that is the only reason. 20 million records since 1990 is gaining a lot of new fans. I wish they would put their records on iTunes though. I have heard the reason is they think the albums should be heard as a whole, but that seems to be a cop out. Apple can certainly require you to buy the whole album. Seems like it would be worth it to Apple to cave in on that to add Zep, because unless they buy Apple Records they are never going to get The Beatles on the store.