Several reasons really. Hyenas compete with lion prides for food, hyenas would eat lion cubs (as well as lions themselves) given the opportunity, and hyena packs are generally larger than lion packs. Lions would see hyenas as a threat to their territory, especially since where one goes, so go others.

Why are lions and hyenas enemies?

Competition between hyenas and lions for resources leads to infanticide—the practice of killing each other's young. Learn why this behavior makes the two species "mortal enemies."

Why do hyenas kill lions?

Hyenas are very intelligent scavengers and they can sense old and weak lions. As a result, they will benefit by using their clan power to kill and eat a lion. Sometimes to cut off the competition, hyenas would also kill and eat lion cubs.