Because Christianity in North Africa, while dominate in the 7th Century, was very fragmented.

Wave after wave of heresy decimated the Spiritual Authority of the Roman Orthodox Church [The Church of Constantine the Great] and so when the Arabs came and conquered it, in the 7th century the locals quickly abandoned Christianity for Islam and that is why you don’t see a sizable Christian minority in Algeria, Tunis, Morocco, or Libya.

The story was different in Egypt, where the populous was unified under Coptic Christianity and we’re led spiritually by the Coptic Patriarch of Alexandria, which is why Egypt was majority Christian all the way till the 1700s.

Because Christianity in North Africa while dominate, was very fragmented.

Wave after wave of heresy decimated the Spiritual Authority of the Roman Orthodox Church and so when the Arabs came and conquered it, the locals quickly abandoned Christianity for Islam and that is why you don’t see a sizable Christian minority in Algeria, Tunis, Morocco, or Libya.

The story was different in Egypt where the populous was unified under Coptic Christianity, which is why Egypt was majority Christian all the way till the 1700s.