Let us say I am the King of France, and I rule over various duchies that it would not be utterly ridiculous to declare kingdoms - Burgundy and Aquitaine, perhaps even Normandy. What stopped me from having these duchies elevated to kingdoms and my kingdom to an empire? Who was in a position to deny me the right to make this decision? If it was the Pope, then surely during one of the schisms a sufficiently compliant rival Pope might have been located. So was it the Holy Roman Emperor who would not have countenanced a rival? Or was there a different constraint that I am simply not taking into account?
I wrote my master thesis on legitimacy in medieval times, so I'll try to explain as much as I can. If sources are needed, I need to look them up and if i make some mistakes, sorry, it has already been 3 years ago since I finished my thesis.
The first question you need to ask yourself is why do you want yourself to be proclaimed emperor? You already are in a higher position than the duchies of french, why elevate them to kingdoms?
The easy answer is prestige. So let's say you have a perfect relationship with your vassals and they accept it. The point is that the other rulers of Europe accept it aswell. They won't.
If you do want them to accept your title as emperor, you need to understand a little bit about the title itself. First, the title is of Roman origin. After the western half of Roman Empire fell, the Eastern Roman emperor was the sole ruler with that title. Till 800, when there wasn't an emperor but an empress. The Pope and Charlemagne decided the title was vacant and so Charlemagne was proclaimed emperor aswell. So if you wanted to proclaim yourself emperor (and you lived in the West), you needed the title to be vacant.
Second, both in the West and East you needed the highest religious man to become emperor. So either the Patriarch if you lived in the Byzantine Empire or the Pope in the West. Without it, I doubt if other rulers would accept your new title.
Of course there are other factors that are important but I think these are some of the most important reasons.
In the medieval era, let's say, the XIII century, the concept of "Empire" is not the same concept used by the European nations during the Victorian era. The Empire was a single concept, it was the succesor of thr Roman Empire itself. The Frankish dominated plenty of territory in the 7th century, engulfing modern France, half of Germany, Switzerland, the Low Countries. Why then, weren't the Merovingians emperors, like Charlemagne? Because he had the papal legitimacy. The Pope, being the Bishop of Rome, had the power to invest a monarch as "Emperor", the thing he did with Charlemagne. Being Emperor didn't game him special rights or powers. It was just a prestigious title, to rival the claim made by the Eastern Romans.
After the Carolingian period, the notion of the western european Empire was the same as to call the Holy Roman Empire, already established. There was no practical reason for the French king to call himself an Emperor, specially within the very des-centralized french political system, where the king was just the first between equals, and ruled based on a series of alliances and compromises with the other french princes.
Source: The Inheritance of Rome, by C. Wickham.