why are jungles mainly in the southern hemisphere?

Answer 1

A technicality. Jungle is just a word for a specific type of forest. The plants in southern regions developed as they are for two reasons. 1) Climate 2) Animals. The way that the plants have interacted with animals and their environment is what causes them to change.

All a jungle is is a forest with "tropical plants" growing in it - as opposed to an Evergreen forest.