Why was the Frankish kingdom declared an empire?

At the end of the 8th century, the Frankish kingdom was the most powerful in western Europe. Charlemagne was able to conquer many lands. His state stretched from Pannonia to the Atlantic Ocean. To consolidate his influential status, he was crowned by the Pope in 800. Thus, he received the title of emperor. In this connection, the state began to be called an empire. However, the Frankish Empire lasted only 43 years. Then it was divided among themselves by the heirs of Charles.