Their new iLife and iWork apps require Mavericks. They don't make money off of iLife, as I suppose anyone who has a Mac can upgrade to the new versions for free (not sure about older versions of iLife), but with iWork, they can't charge new customers for the applications unless they're on Mavericks. Could be a reason, probably not the reason.
Also, I feel as if much of Mavericks improvements are under the hood. What Apple is offering as new user features are largely things that other players provide for free on the Web. For example, Maps is something that you can get the equivalent of for free on the Web. iBooks is something that is already free on iOS, and the idea that Apple would make you pay money in order to get iBooks you've already paid for onto your Mac might not go over well with people. Amazon doesn't charge so that you can get Kindle on iOS or on the Mac. And Apple may also have an interest in more people buying iBooks (although, to be honest, I can't see many people buying iBooks on the Mac).
Finally, I think in the old days, Mountain Lion and Mavericks might not have warranted a whole point update. I sort of look at both Mountain Lion and Mavericks as big .1 updates for Lion.