The second reason for mass illiteracy was to maintain control.
Education is power.
Educated people are potentially dangerous. Even in the former Soviet Union, which provided universal, free education through graduate school, the government was very careful to control the material that was taught. Ignorant people are far easier to control. We know it today, and people knew it thousands of years ago, too. The Greeks and Romans would have been very different people if, per impossibile, they had achieved mass literacy.