Bai Farama gave one of his sons to these priests after he had become Catholic and this boy was taken to Europe and educated up to university. He became Bai Farama Boure from 1606 to 1630.
This was about the time of the start of the Trans-Atlantic Slave trade. So, when the slave trade started and with connivance of some local chiefs the trust between Europeans and the local population was lost and that informed on the late acceptance of western education or literacy.
Another factor is Islam. The people understanding that education as it is brought to them by foreigners, is about the three Rs, Reading Writing and Arithmetic, saw little need to take the strains to undertake another exercise of learning the same which Islam offered.
Finally, because of the independence of mind and self-reliance of most northern people they saw little need to use education as a channel for human resource development taking into consideration that they have traditional skills they can use, at their level to live their lives.
So, to say they are more illiterates in the north is a logical fallacy because there are many who read and write in the Arabic and do accounting etc in the same.
However given the fact that we have made little efforts to institute using our languages as medium of communication in the school as Ethiopia and much of east Africa does or even India, Malaysia, Indonesia, Vietnam, etc. we can find a way to get northerners to go to school ,free or otherwise by another pronouncement than to say they are illiterates. They can say same of anyone who does not read or write Arabic.