That's a hard one to answer but from all my reading it would seem there was one main reason. The Berbers were nomads so they never were able to be collectively together to form an army against invaders. Therefore, the Berbers were constantly being conquered by someone. Each time they had to learn new laws set out by the conquerors. When the Arabs came with their mighty army, they knew they couldn't fight so giving in was in an odd way a protection system of kinds.