Why did the Byzantine Empire initially view Charlemagne as a usurper?

The Byzantine Empire viewed Charlemagne as a usurper because they considered themselves the only legitimate successors to the Roman Empire and the throne of the Caesars.12

From the Byzantine perspective, the imperial title was not something that could be created or bestowed by the papacy. Consequently, they regarded the coronation of Charlemagne by Pope Leo III in 800 as an extralegal and revolutionary proceeding. Because the Byzantine emperors maintained their claim as the sole true heirs to the Roman tradition, they viewed Charlemagne’s assumption of the title "emperor" as an illegitimate challenge to their authority.1

Furthermore, the Byzantines believed that the pope, as a subject of the Byzantine emperor, lacked the constitutional authority to confer such a title. It was not until 812, following years of negotiation and conflict, that the Byzantine emperor Michael I grudgingly recognized Charlemagne’s imperial title, though he notably did not recognize him as "emperor of the Romans," thereby preserving the Byzantine claim to be the only true Roman successors.13