I've wondered about that. There's at least one other reason why the back tire would wear more quickly: it applies the forward motion. While the front tire can simply roll, without significant forces along the direction of movement, the back tire is applying a backwards force to the road surface at any time power is applied to the drive train. Ideally, it's just a contact force applied through the elastic material of the tire. But if there's any slippage (and I would expect there to be, on a microscopic level, even ignoring the instances of loss of traction where the whole tire is obviously slipping), then that adds friction as a mechanism of wear. I would expect friction to be worse than downward force. Front tires experience significant friction only during front-wheel braking and in turns; rear tires whenever the bike is powered forward (as well as during rear-wheel braking and in turns).