Notwithstanding (slightly dubious) recent rumours of a new video codec capable of pushing out 4096x2160, iTunes today can only offer high definition video in 720p. To move that to 1080p would require 2.25 times more disk space on the iTunes servers and the same increase in data bandwidth between those servers and the paying customers. Those are not cheap investments.
In turn, consumers would see a similar increase in streaming requirements. Xbox Live can stream 1080p video, but it requires 8-10meg broadband, which leaves an awful lot of people out in the cold. It has the option of downloading instead, but if you're out in the sticks on a 2meg stream you're looking at more than eight hours to download your film at 1080p. You'd best plan your Friday night viewing before leaving for work on Friday morning.