Northern Africa was home to many religions before the Islam came. There were lot of pagan religions, Christians, and Jews. This all rapidly changed though with the introduction of Islam.
The Islam came with the Arab conquerors. The Arabs were all united with the new religion, while Northern Africa was in a mess and there was not major force in the region. The Arabs used this and attacked, managing to conquer this region with relative ease.
Once the Arabs conquered Northern Africa, they immediately the Islam as the official religion, and there was no tolerance toward the other religions. The people in Northern Africa were forcefully converted to Islam, lot of them killed because they didn't wanted to convert, were persecuted, or taken as slaves. On top of it, lot of Arabs migrated into the region, so Northern Africa quickly became Islamic region and the other religions were almost totally eradicated.
