One state I could see creating this canal system is the Berbers if Aksel/Caecillius successfully repelled the Arabs or if Dihya never used the scorched earth tactic causing her to lose the support of urban populations. Following the defeat of the Arabs, the Berbers of Tunisia and Algeria could coalesce into a loosely united state. After a few decades of further unification due to wars against the Byzantines, the king of the Berber confederation could invite West European lords to kick out the Byzantines and once the Exarchate has been seized, the king would be able to crush any powerful tribes and centralize the realm under himself. Then to increase his kingdom's prosperity, he begins construction of the canal system. With defences constructed along the system, he is able to hold back further Arab incursions into the region.