My first question is how do you know? No one here knows the extent of the prototype that DICE developed, as far as I know. They could have easily had the animation rigging done already - that alone, really, is what's substantial about the game and its most complicated part. Making a character walljump is nothing new. Making the character walljump realistically is a bit more of a challenge. Aside from the play style, there wasn't all that much about Mirror's Edge that was difficult from a game design perspective - no huge amount of characters on screen, no fancy AI, and the graphics, while awesome in their way, weren't computer-killers.
Plus, the game used UE3, which is now available to most people via the UDK (Unreal Development Kit). Give some enterprising people some time with the UDK, a sound studio and some animators and maybe a mocap system, and you could probably get something like Mirror's Edge released pretty quickly. I mean, look at what http://www.moddb.com/mods/mirrors-edge-source-trailer-map did in the Source Engine. There are also groups looking to develop a Mirror's Edge mod / game in the Source engine.
Because that is what [b]makes sense]/b]. We haven't seen anything from the game, which leads me to believe that development hadn't progressed far enough to have anything that would represent the end product. If they did have it progressed that far in such a relatively short amount of time, why would EA cancel it? They could just let that small team keep working and finish within the year if it were as far along as you people seem to think. Even if they have all that stuff you say they could have, that's not enough to make an official announcement out of, and they can keep that around until later on.
What I think happened is that they had a small team working laying the groundwork for the development of the game. Maybe they have some animations and models done, a story lined out, maybe even a few test levels. They then presented that to the executives so they could determine whether or not they wanted to go all out and commit to the development of the title, which surely would have needed more people in order to complete. They just decided that, right now, it makes more sense to commit fully to Battlefield 3. After that's over, they can come back to whatever they already have and make a full game. Maybe even use Battlefield 3's engine for it.