One big reason is because the G1X does not have a mirror box. Thus the flange focal distance is much shorter than the EOS DSLRs, allowing smaller lenses to be built. But a side effect of shorter FFD is light fall-off at the edges due to the angle which the light is hitting the sensor. That can be corrected by (expensive) micro-lenses, which Leica is using on its huge full frame (Kodak made) sensor.
2012 is going to be an interesting year in photography. First EOS 1D-X, then Nikon D4 and D800. Next thing you know, G1X and X-Pro1. I can't wait to see what else is in store.
The Pen Pro?