The main cause is poverty and lack of education. They're very poor, so they can't afford school. If the children do go to school, they drop out very young to work and help earn money for the family. That's not the kind of cycle people can break out of easily.
Add to that the civil war from 1960 to 1996 where the military persecuted the native people, killing up to 200,000, and they've had a pretty rough time of it.
