I would think on of the reasons is because the food that mice eat has a greater variety, in the sense that while a lion may only eat meat and have to catch its prey the mice can eat without relying on other animals. Lions also have more competition between other animals like them for example cheetahs and leopards on the other hand a mice is more independent. On period where food is scarse mice can get on with eating what the earth provides like plants lions can’t.
