What happened to delta updates?

It was not too long ago (turns out, it was four years ago) that it was announced* in an Apple keynote presentation that we will get delta updates from the Mac App Store with the next OS X update (10.7 / Lion). That delicious feature would greatly reduce the number of megabytes which need be downloaded every time an app is updated, which could, ultimately, save many hours of time we otherwise spend waiting for said downloads to finish.

Since then, I’ve only witnessed such delta updates a couple of times, but mostly on the iOS App Store. My question is: Why is this still not a reality in a generally sophisticated OS? Unix systems even come equipped with the necessary tools (diff & patch). Server storage space can’t possibly be the problem here, considering the amount of extra data sent to thousands of users every day has to be more of a drag than buying a few additional terabytes worth of HDDs. (Excuse my ignorance about data centers; it’s likely more than just a few terabytes…)

Delta updates would be especially convenient for OS updates. I just don’t get it why every minor update – be it iOS, OS X or Xcode (which should really be split into multiple apps again, because right now, it’s unnecessarily heavy) – weighs in at multiple gigabytes every time, even though I don’t actually need any of the fixes contained in the update (but I do want to get rid of the badge telling me to update, because badges are kind of stupid).

I do understand, though, that most deltas can’t be kept on file, since the number of version combinations would grow exponentially over time but I don’t see any reason not to create patches for the last two versions.

* Engadget article from 06/06/2011