Why was Charlemagne not truly the "Emperor of the Romans"?

Answer 1

Charlemagne became king of the the Franks when he was 23 years old. Later in his career,
Pope Leo III crowned him Holy Roman Emporer in 800 C.E. It was more a religious title or honorarium than a gift of lands. Charlemagne ruled most of Western Europe, but wasn't able to consolidate the Italian penisula into his holdings. Thus the Roman empire was alowed to maintain it's downward sprial while Charlemagne brought more learning and Christianity to the Gallic and German reigns.

Answer 2

Well for one. The Roman Empire didn't exist when he came to power. He was the leader of the Germanic Tribes, which he united and controlled Europe.