Because the snow would melt due to global warming, the ground would be brown and dirty. The brown rabbits would now blend in. The white rabbits wouldn't. Prey would eat the white rabbits because they can't hide well against the brown ground. The brown rabbits could hide and then, when more of the brown rabbits mate there would be more brown rabbits. Basically natural selection. Less white rabbits because they can't hide on the brown dirt so they get eaten and more brown rabbits because they can hide and survive not being caught by the birds.