what factors cause so many fast food restaurant s to locate in South Los Angeles?

Answer 1

demand...as in supply and demand.

Answer 2

Population density. The more people per square mile, the more potential customers.

Since there is a trend in consumer to eat out from fast food providers, the economic sense of building new stores is there. The question of where to place them will, then, be answere by figuring out where the customers are.

More people = more customers.

Add to that the mobility of the consumer, which gives them accessability to the restaurants. Where is there a more mobile population than in the Los Angeles Area?