I think normally it has to do with weight balance and where the optimal amount of braking can be obtained. For weight balance at one point, F1 cars had the calipers down at the bottom of the rotor on the front wheels, making for a lower COG perhaps. But now, they have them more towards the rear of the rotor, problaby inducing more braking force. So it seems the advantage of having that slightly lower COG really wasn't enough to keep the calipers on the bottom and it wasn't creating as much braking force as they do in the rear of the rotor. Thus you see the calipers on the rear of the front discs.
I may be off somewhere in that, but I'm sure somebody else will be able to correct that.