One of the reasons for their easy success was probably the fact that the Roman peasantry was suffering badly under the authority of the empire, it was levied with taxes so seriously, and the debts were so great that the peasants were ready to welcome any change. The Eastern Empire not only survived these attacks, but persisted for centuries, despite the constant struggle that it had to wage against the Arabs, and later against the Turks (Cameron, 2011).
Cameron, A. (2011). The Mediterranean World in Late Antiquity: AD 395-700. Routledge.