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

The Frankish state, which was successfully ruled by Charlemagne, like the Arab Caliphate collapsed because the power of the supreme ruler (emperor or caliph) weakened under the influence of court intrigues and disobedience of the rulers of the conquered peoples. Because both states consisted of different ethnic groups who wanted independence, there was a decentralization of power, supported by wealthy feudal lords. Local feudal lords were able to create their own army and overthrow the supreme power, or they recognized it only formally.