Nellisir
Hero
Since they decided not to introduce yet another character, one that would need MAJOR continuity work to fit into the movies, to be the macguffin. At least Kitty Pryde a) was the original time traveler in the original storyline, and b) the explanation (which I never heard, so maybe I made up) that she can phase people's consciousness through time as well as through solid objects makes sense for about two seconds, which is 1.5 seconds longer than anything else. (Colossus: I smash people through time...I think. They never wake up.)Edit: Since when can Kitty Pride help peopel travel through time?
Can you imagine the nightmare that Rachel Summers-Grey would create as a movie character? She's the daughter of two dead characters, with no prior movie existence, with copycatted powers, a messed up backstory, and only there to shove the plot along.
Having Kitty do it is deus ex machina, but it's better than any other alternative.