Hi, this is just off the top of my head, so please let me know if you disagree or have questions:
One explanation is that white fur is a dominant trait, whereas black fur is recessive. As they breed, the recessive trait slowly dies out. Another explanation is that the white fur is better camouflage from predators for their environment. Through survival of the fittest, rats with white fur begin to outnumber the rats with black fur.
