For the longest time there were only the Frankish and later Holy Roman Emperors (excluding Roman Emperors in Constantinople, Trebizond, etc.).
Only starting in the 18th century, there were the Russian Tsars, and later along with Napoleon, the Austrian and German Empires. Why did no earlier Kings (like for example the French or English, especially in the wake of colonialism) for example declare themselves Emperor?
The Donation of Constantine. When Charlamagne was going to be emperor, it was very much in doubt how he would go about getting crowned. Eventually in a ceremony the Pope put a crown on his head and said he was Holy Roman Emporer. Some people state this surprised Charlamagne.
Do you see what a trick it was by the Pope? This was the Pope stating HE had the power to crown emporers and not the emperors themselves. How did the Pope get this power? The Donation of Constantine. It was a forgery but it was basically it was a will like document that said in substance "I,Constantine, Roman Emporer, have split the Empire into East and West halves, and in the West only the Pope can crown emperors." However goofy it was, the Pope could still excommunicate folks so people went along.
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