Do you mean why as it called the Grand Duchy of Tuscany instead of the Kingdom of Tuscany? It was called the Grand Duchy of Tuscany instead of the Kingdom of Tuscany because the ruler of Tuscany was the Grand Duke of Tuscany instead of the King of Tuscany.

Why was the ruler of Tuscany the Grand Duke of Tuscany instead of the King of Tuscany? Because grand duke was the highest title he could get the Pope and the Holy Roman Emperor to let him have.

Since Tuscany was part of the Kingdom of Lombardy or Italy and the Holy Roman Empire, if the ruler of Tuscany claimed to be a king he would risk having the Emperor send an army to conquer Tuscany and execute him for treason.

There are only a few cases in European history where nobles promoted themselves to king without permission of higher authority. They include the King of Portugal, the King of Sicily, the King of Burgundy, the King of the other Burgundy, etc. Andin those cases it may have taken years or decades to get any kind of international recognition.

As a general rule, a titled nobleman needed a decree and document from his king to get a higher title, and his king would not be willing to promote him to king.

In the 19th century there were independent rulers using noble titles like Prince, Duke, and grand Duke. And they didn't call themselves kings because they were not authorized to call themselves kings or recognized as kings by international diplomacy.