Because Charlemagne changed the world of that time and without him our present would certainly be very different.
The Lombards (and their king, Desiderio) probably would have kept much of Italy for who knows how many more years, perhaps the Church would have disappeared, the birth of continental European nations would probably have been delayed for centuries and the balance (and even the current problems) would be totally different. But this can always be said, speaking of an event of the past.
That of Charlemagne is a story of which we often only know the aspect relating to his coronation as Emperor, which took place on the night of 25 December of 800, but his life is all a tangle of strong powers: Church of Rome, Empire of Oriente (led by Irene), the new Western Empire that begins with him.
It is a period of struggles for conquest, of fratricidal struggles between peoples of the same blood, religious misunderstandings, iconoclasts and iconodules. It is the time when the struggle against polytheism in Europe is the justification for every kind of cruelty. Charlemagne uses all his power and strength, since he was still a boy, he had the opportunity to see Pope Stephen II appear as a suppliant at his father's house to ask for help.
Stefano II, Adriano, Leone III ... are just some of the characters in this story like Rome, Byzantium, Hereburg, Aachen, Mainz are just some of the places where it was made.
But that's enough, I don't have to tell you the story and life of Carlo.
I invite you to read the biography and immerse yourself in those ancient, sometimes cruel times, part of our common past.