To take a wild guess... It would be so they could advance the communication protocols between lens and body, without changing the mount. Perhaps fancy AF motor or aperture control? Or probably more likely, move the sensors and processing for IS into the body. That would gain you increased processing power for better algorithms ("scene" linked IS modes for consumer bodies?), better sensors which are farther off axis (should be more sensitive). Servo AF tracking could also possibly benefit from that sensor information in some instances, maybe, couldn't hurt.
The only benefit a wireless connection has over an electrical is that you can completely seal it against water and dirt. But when this part of the camera gets water, the contacts are your smallest problem.
In terms of simplicity, speed, power efficiency, reaction time, interception, etc... the electrical are way better. And specially because a lens needs a lot of power to focus, a wireless connection would have to be pretty complicated.
And about the compatibility point: whatever data signal you can send through the air, you can also send it through an electrical contact. There's no problem in increasing the functionality and still keep it 100% backwards compatible. USB and ethernet are examples.
No, I think the only reason for Canon to change the mount is to make people buy new lenses and cameras, and to prevent companies like Sigma from releasing lenses for Canon cameras.