One possible explanation for the increasing number of white-furred rats could be that their coloration offers better camouflage in their specific habitat, which may have more light-colored substrates like sand or pale vegetation. This increased concealment allows white-furred rats to evade predators more effectively, leading to higher survival rates and, consequently, more successful reproduction.
Another reason might be related to genetic factors and environmental conditions. If the population has a higher frequency of genes associated with white fur, and if these genes confer any advantages in colder climates—like better insulation—this could lead to a natural selection process favoring the white-fur trait over generations.