What are the common reasons for the collapse of the Caliphate and the empire of Charlemagne?

In the 8-10th century, the Arab Caliphate and the Frankish kingdom had a number of similar reasons, due to which the collapse occurred. Both states were conquered by barbarians and held on to the strong power of the monarch. They united people with different cultural customs and preferences under their rule. They strove for self-determination and independence. Also, a special contribution to fragmentation was made by the presence of many heirs. Because of this, fragmentation manifested itself. The countries lacked clear legislation and controls.