For centuries in the Middle Ages, in many regions of Europe there were not kings, but dukes, archdukes, etc. For instance, in Italy one had the Archduke of Tuscany and the Duke of Piedmont (until 1720) or the Duke (Doge) of Venice, etc. They were completely autonomous entities, but they were not kings. Apparently in some cases, this mattered a lot: the Duke of Piedmont gave up Sicily in 1720 in order to get Sardinia, with which he could finally be considered King of Sardinia (which included Piedmont, Liguria, Savoy etc).
My question is: What prevented the Dukes and Archdukes from saying, "well, from tomorrow, please call me a king"? Why was becoming a king so important for the Duke of Piedmont? What could he do as a king that could not as a duke?
I know that in theory the Holy Roman Emperor was his master, but as far as I can see, the German emperor had no practical influence on most of these states for centuries and I doubt he would have had anything to say if the Duke wanted to be called a king.
Did the Pope have influence on this? Maybe they really would not have accepted it and would have moved in with their troops. If so, why? Is just being called a king more threatening for the emperor, the Pope, or the other states, than being a duke?