Why did the person in charge of a principality not just declare themself king?

I don't understand the concept of principalities, why wouldn't you just call yourself king ? The job description appears to be exactly the same.

Sameness and simplicity are completely different notions, and it is quite common for the same (complex) concept to embody various aspects. Thus, for instance, the job description you mention implies several distinct qualities :

• occupying the first position in a hierarchical structure; for expressing this idea, the Latin princeps (prince), Greek archon (monarch), and German Fuerst are spot on.

• leading the army into battle; for referring to this important role, the Latin dux (duke), Italic doge, and Slavic voivode are more appropriate.

• commanding one's subjects; for denoting this, the Latin dictator and imperator (emperor) are more suitable.

• ruling over the land; the Latin rex and Sanskrit (maha)raja being prime examples of titles related to this specific task.

• boasting of a Caesar-like position; for reflecting this noteworthy characteristic, the Latin caesar, German Kaiser, and Slavic czar are usually employed.

As one can clearly see, the above list contains quite a lot of Latin-derived terms. But what if one prefers a more native title ? Well, as it turns out, the English king, German Koenig, and Slavic knyaz are cognates !

I hope this will help create a better understanding of the meanings behind the many historical titles.

Several people did just that: have control over a territory, declare themselves to be $insert–title.

Odoacer declared himself to rex Italiae, king of Italy

Fall and death
As Odoacer's position improved, Zeno, the Eastern Emperor, increasingly saw him as a rival. According to John of Antioch, Odoacer exchanged messages with Illus, who had been in revolt against Zeno since 484.

Napoleon

To expand his power, Napoleon used these assassination plots to justify the creation of an imperial system based on the Roman model. He believed that a Bourbon restoration would be more difficult if his family's succession was entrenched in the constitution.

Wilhelm I

William I, German Emperor

Instead of Emperor of Germany! The feelings of other princes in Germany were an obstacle, and other European rulers or countries as well.

They all have in common that it was difficult for them to not come across as parvenus. Upstarts make for jealous peers and neighbours, all keen to not fall behind or loose their standing.

That is, it all depends whether it is perceived as something real.

The Emperor of the United States began his rule with well formed reason and conviction, after all:

By 1859, Norton had become completely discontented with what he considered the inadequacies of the legal and political structures of the United States. On September 17, 1859, he took matters into his own hands and distributed letters to the various newspapers in the city, proclaiming himself "Emperor of these United States".

If one can pull it off, fine. It looked for a while that Napoleon could do it. Wilhelm was also in a position to hold on until his death.

It's difficult enough to subjugate your own subjects, many kings and Roman or even Holy Roman Emperors knew the lyrics of that song by heart. But during several epochs, usurping the wrong title could mean imminent attack by rivals. Rivals who showed them who was king.

Sometimes sovereigns had to downgrade their title. We often think of Franz Josef as Emperor of Austria-Hungary after the creation of the dual monarchy (with Hungary having won autonomy from Austria in 1866-1867) since he previously had been Emperor of Austria. But in fact he was Emperor of Austria and King of Hungary, complicating many things...e.g., the joint army was called "the imperial and royal army" as the Hungarians insisted it not be called "the imperial army".

In another vein, it's worth remembering where the title "prince" comes from...the Latin "princeps", originally meaning something like "first citizen" or "first man", which Octavian first used as a political title, specifically to avoid calling himself a king. So, "to avoid being stabbed by angry politicians like Julius Caesar had been" would be a flippant answer to your question. (And slightly inaccurate, as the political regime Octavian created is generally called the Principate and was not called a principality, and as Julius Caesar hadn't really allowed himself to be called king, but everyone was afraid that he would be. There was a lot more to Octavian's convincing people he wasn't aiming at kingship beyond the name "princeps"--that was just part of it; the "restoration of the Republic" was crucial as well.)

Expanding on Alex' answer, there is the example of the Hohernzollern dynasty. They were the prince-electors of Brandenburg, but from 1701 to 1772 they called themselves kings in Prussia. They did not call themselves king of Prussia yet because Brandenburg was part of the HRE and their kingship was only seen as valid outside the HRE -- in Prussia, not in Brandenburg.

After some generations they got the other German princes and kings to accept their kingship without the qualifier.

This thread doesn't answer my question as principalities weren't under a king who had given them their title and who they owed fealty to. Principalities are top of their own pile.

The original question is wrong in claiming that a principality was an independent state. There have been some independent principalities but the majority of all principalities were dependent states, parts of higher and larger states. And it has often been rather controversial whether a specific principality at a specific time should be considered dependent or independent.

In the Holy Roman Empire the princes were a class that arose by about 1200. A prince or Furst was the first man or ruler in his principality and collectively the princes or Fursten were the first men in the empire. Some of the princes had the title of prince or Furst but others used other titles.

From lowest the highest the titles used by the princes in the Holy Roman Empire were princely count, landgrave, margrave, count palatine, prince, duke, grand duke, and archduke.

And the princes of the empire were all vassals of the emperor, and their principalities were all within the empire, until 1806. There were some independent principalities, duchies, and grand duchies in Italy and Germany after 1806.

[Added 01-26-2020. And in the case of the German princes, dukes, and grand dukes, their states were all only semi independent, Napoleon established the Confederation of the Rhine in 1806 right before the Holy Roman Empire was dissolved, and all the German states except Austria and Prussia eventually joined it. And it was probably Napoleon who awarded higher ranks to various members of the Confederation of the Rhine. All of the surviving German principalities were only semi independent after 1815, being members of the German Confederation established in 1815, and so their rulers couldn't simply promote themselves to higher ranks without consulting the other members of the German Confederation. The German Confederation was dissolved in 1866, but most of its members joined the North German Confederation of 1867 to 1870, and the South German states joined in 1871 when it was transformed into the German Empire. So only the Italian principalities were totally independent states.]

The independent princes, dukes, and grand dukes after 1806 didn't have the power to use any title they wanted to but had to use title awarded or recognized by the other nations of Europe. For example, when the deposed elector and Landgrave of Hesse-Cassel was restored to his throne after the Napoleonic Wars he wanted to take the title of King of the Chatti but the Congress of Vienna only awarded him the title of grand duke, but he continued to use the title of elector. It is claimed that the Congress of Vienna recognized and granted the title of king to rulers who had been electors before 1803 and recognized and granted the title of grand duke to rulers who were made electors in 1803 and some other rulers.

In Russia, Kievian Rus is usually called a principality. But of course there are several possible ways to translate the title of the ruler. The title of the rule was knaiz which is usually translated as duke or prince, but could be translated as king. Kievian Rus was soon divided into several principalities under the authority of the ruler of Kiev itself, who became known as the velikiy knaiz which is usually translated as great or grand prince or duke, but could be translated as great king or grand king.

There were somewhat similar situations in Medieval Lithuania under a ruler usually described as a Grand Duke, and in Medieval Poland which for centuries was divided into several principalities under a senior prince. When Christian rulers used the Latin titles of princeps "prince", dux "duke", magnus dux "grand duke", etc., it is certain that they were not claiming to be kings, but when pre Christian rulers used titles in their native languages it is less certain whether those titles should be translated as prince or king.

Examples of nobles who proclaimed themselves kings include:

In 879 the Frankish noble Boso was elected king of Burgundy or Provence by a a gathering of Nobles.

In 888 Margrave Rudolf was elected king of Burgundy by a group of nobles. The two kingdoms of Burgundy united to form the Kingdom of Arles or Burgundy in 930.

Who was king of Sicily before Roger II? Nobody. Roger II, Count of Calabria and Sicily, was crowned King of Sicily and Italy in 1130.

Count Alphonso of Portugal in the Kingdom of Leon in Spain proclaimed himself King Alphonso on 26 July 1139, and was recognized as king by Alfonso VII of Leon and Castile 5 October 1143 in the Treaty of Zamora, and was recognized as king by the pope in 1179.

I note that in those cases the nobles in question did not simply declare themselves kings but had to do a lot of political effort before and after becoming kings in order to be recognized as kings.

It is possible that experts on European history will think of other examples of nobles making themselves kings.