Because the PC version will probably look the best anyway.
Obviously. However, the point still stands: If you want this to be a next gen engine then it should be able to run on a next gen platform. PC games have mostly been console graphics+options refine visuals even more+non-restricted framerate.
PC might be generation agnostic, but technology isn't.
I could understand their desire to show off BF3 on PC first given how large the gap had become between existing HD consoles and PC tech by 2011. But now, it is best to show off the baseline where PC players with sufficiently powerful hardware get to push certain visual settings to 11.