Some of the major causes are:
Economic condition of the people (poverty). So children need to work and can't be spared for school. Or schooling may only be compulsory in elementary grades. Or there may be no free education.
Ineffective implementation of educational programs in rural areas.Lack of entrepreneurship by the educated to share and spread knowledge.Preference to work rather than to study (no internal economic incentive)Emigration of educated individuals (external economic incentive)
The governments in many developing nations lack either resources, expertise, or the incentive to improve educational opportunities. The available resources cover only a tiny fraction of the populations that would benefit from them.