The reasons why cats have low white blood cells are as follows:

1. Feline distemper virus infection: Low white blood cells in cats may be caused by feline distemper, which generally manifests as fever, vomiting, diarrhea, etc.

2. Bacteremia: Bacteremia will cause a large amount of white blood cells to be consumed, which will lead to a decrease in white blood cells in the blood.

3. Tumor: Certain tumors cause disorders in the production of white blood cells, leading to a decrease in white blood cells in the blood.

4. Acute enteritis: Low white blood cells may be caused by acute enteritis. Acute enteritis is contagious and requires isolation during treatment.

5. Lack of nutrition: Cats can also suffer from low white blood cells when they are lacking nutrition. Just give the cat enough nutrition and rest more.

Note: When it is found that the cat’s white blood cells are low, it is best to check whether it has a disease immediately to avoid delaying the condition and causing life-threatening consequences.