The Merriam-Webster dictionary defines ignorance as: "lack of knowledge, education, or awareness," of which we know that ignorance is most definitely a form of slavery, as during the antebellum days, a North Carolina statue was passed which stated in part, "Whereas the teaching of slaves to read and write, has a tendency to excite dis-satisfaction in their minds, and to produce insurrection and rebellion…" so that, it was actually a crime in that State, as well as implicitly or explicitly a crime in all slave-owning States, to teach a slave to read and write. So too, in all the ages preceding modern times, it was most common for literacy to be limited to the domain of the Church, the rulers of the state, such as the aristocratic class, and certain members of the common people, such as doctors, lawyers, as well as other well placed businessmen and traders of goods.
The most significant reason that ignorance existed and was so prevalent in the history of mankind, is that those that were its temporal as well as its spiritual leaders, found it beneficial for themselves not to have to compete against alien ideas or free minds, for those that are ignorant are in rather poor shape when it comes to rebellion, governance, and civilization, for they lack the skill-set to successfully pull their people together into one cohesive and competent unit.
This meant, that the masses of people were primarily treated as beasts of burden, of which, their productivity and output provided not much more than a bare means of subsistence, with first rights of their productivity actually being taken by the state or the owner of the lands. In addition, when it came to war and battles, the masses were first in line to serve their country, and thereby it was their blood, and their bodies, that were in the forefront of battles, of which, rape and pillage were the rewards that these troops received for their combat.
A society in which the rulers, along with a privileged few, were able to rule the masses by virtue of not only owning pretty much everything in sight, but also being essentially the owners of the written word, mathematics, and all the advantages that literacy provides to those people that wield it, made for a world in which, those that were ignorant, were, if not slaves in name, slaves in fit, form and function.
If we then fast forward to modern times, it is easy to recognize, that if one is functionally illiterate, that is to say, that person can't readily read, write, or do the most basic of mathematics, not only is that person's employment skills reduced essentially to the strength and brawn of their body, but in all of the many and myriad interactions in which one needs some sort of grasp of literacy to make competent and well thought out transactions, they are at the mercy of those that are literate, and such consideration that they wish to grant for those that are illiterate.
To be ignorant, in a world in which the multitude are not, is to live at a severe disadvantage, and at the dictate of others, of which, those that are unable to fend for themselves because of such ignorance, are enthralled to those that do.