technically itv hd isn't a channel. its an interactive service, and until itv make it into a proper channel sky wont be able to utilise it.
there's all sorts of rumours floating about on the dedicated satellite forums as to why itv have gone down this road, but who know's.
lets not forget though that when sky launched it's digital service back in the late 90's, itv wasn't in the original channel line up, even though the 103 spot was reserved on sky for them