You asked the million dollar question there my friend. We may never know what compelled the people of the Maghreb to convert to Islam so quickly while most people living in rural areas of the Levant mostly remained Christian until the Mamluk period. That said, we can see a marked decline in Christian hierarchy over the period from late antiquity to the medieval period. During the Roman period, there were 200 bishoprics in the Maghreb. By the 11th century there were 40, and by the end of that century there were only two.

The Vandals' conversion to Arianism would not have helped matters by any means. Their control of North Africa would have made it difficult for the churchmen of the region to maintain a connection to the imperial capital. Additionally, the Vandals caused the North African economy to tank. This would have made it difficult for the Church to function as well. In The Berbers Brett and Fentress suggest that the return to the Roman Empire failed to remedy the situation as the Eastern Empire that defeated the Vandals was now a Greek speaking empire. This isn't entirely true. Greek didn't become the language of state until the reign of Heraclius in the early seventh century and Justinian was known to have spoken Latin. That said, Justinian's administration was content to let the Maghreb whither on the vine as they attempted to reincorporate the Italian peninsula back into the empire.