Education is free (and compulsary) until 7th grade, but even before then the cost of things such as school uniforms and transportation to and from school are too much for families. After sixth grade the dropout rate grows greatly as seventh grade and up are no longer free to attend.
Some families no only cannot afford the money to send their child to school, they also cannot afford to not have the child work. The family may need help on the family farm or perhaps someone to go into the town/city and sell things like homemade sweets.
In rural areas the options are especially limited.
While having an education does not guarantee employment, it is certainly a huge advantage. Possessing computer skills or being bilingual open up job possibilities. Having a highschool degree (not to mention a college one) can be the road to a much better life.