Why is Islam successful in Southeast Asia, as
opposed to South Asia?

Islam was popular in Southeast Asia because it, unlike previous belief systems could be used to validate a ruler's power through the divine. Islam spread throughout the Indonesian archipelago slowly and the process was generally peaceful.

location is the answer to this

in 550 b.c.e. cyrus the great, the leader of the persians, conquered the medes and united the iranian people under one ruler for the first time. cyrus became the first king of the persian empire and went on to establish one of the largest empires in world.