Because if you ever need to fully restore your device iTunes needs the full ipsw. So I guess apple doesn't want you to download the delta and the full ipsw.
That seems ridiculous since you'll have to download the delta (from your phone) and the full IPSW (from iTunes) if you have to restore an OTA updated phone. Plus, if download bulk is really the concern, most people don't have to restore their iPhones. Apple would surely come out ahead letting users delta from iTunes and then having some of them download the full IPSW....
Hopefully in the future deltas will be on iTunes as well as OTA.