First of all, the word jungle as we use it today doesn’t mean what it previously meant. It used to mean “land overgrown with underwood” which is a place lions could possibly live, as they do sometimes live in habitats other than the savanna, like some kinds of forests.
Also, lions have been known as the king of beasts for centuries, due to both their position at the top of the food chain and their regal appearance. Additionally, the way male lions walk among the females and cubs is reminiscent of the way a king walks among his subjects. The male lions are the guardians of the pride. Their role is to protect the territory and the young from intruders. When it comes to humans, a king is viewed as the guardian of his country and its people, and is at least partly responsible for leading the troops into battle. Lions have long been seen as strong, brave, and proud creatures. These are qualities that many say leaders should have, and because we observe them in lions, we tend to think of lions as leaders.
Reference:
http://www.theguardian.com/notesandqueries/query/0,,-2693,00.html