Why was Dom Pedro I crowned Emperor and not King of Brazil?

I'll preface this by saying that English isn't my first language, so pardon me for any mistakes I might make.

The easiest way I can explain this is going back a bit, to the year 1798 where a man called Rodrigo de Souza Coutinho along with other important people had created a "project", it's easier if we look at this as different projects. His ideas where basically creating a new Empire headed by Portugal together with all it's dominions, the provinces and states would have the same rights and privileges as Portugal, this would serve two purposes, consolidate Portugal rules and also strengthen the monarchy; it would create an civil empire, since Brazil would have a "independent" administration. The issue was that a few years later Portugal was invaded by France, forcing the royal family and its court to move to Brazil, now the Portuguese immigrants needed to establish themselves, not making everyone see them as outsiders, this was done selling titles and giving concessions to the aristocracy here in Brazil, so they would support the monarchy and finance it as well, but it wasn't everything, we also wanted an elevation of status, that's what happened in the year 1815, when you spoke of an Empire in that time, you were talking about Portugal, Brasil & Algarves, that meant we had been elevated as equals, now this is the first project, a luso-brazilian empire

The second project was meant as a constitutional government, where Brazil would have some freedom, but still ultimately be ruled by Portugal, basically having it's own government under Portugal. This one was proposed by farmers and merchants but didn't gathered much strength.

Now we jump ahead to an independent Brazil, why was it considered an Empire? There is no easy answer here, so I'll state a few reasons, the first one is to consolidate the political power under the figure of the monarch D. Pedro I, there was still some folk loyal to Portugal and they had to maintain power, not only that, they had to convince everyone he was the right and divinely selected leader of the nation, you can notice this by his coronation ceremony, where he did rituals and symbolic ceremonials similar to medieval one, wearing a military uniform, to give the idea of a soldier-emperor, someone who would fight for the independence of the country and for it's benefit as well, this was used as a way to take away the idea of dividing the power between the monarch and someone elected by the people, so he could control everything, or at least maintain the facade of doing elections and what not.

Another thing I should point out, the terms of "emperor, kings, queens" are used in a fluid way, think of the British Empire, it was said to be so large the sun would never set right? But it was ruled by a Queen/King, not an Emperor or Empress, so keep that in mind as well, of course it was also used to show Portugal we would not submit (eh we kinda did) and to legitimate the ruler. One more thing is the continental size of Brazil, that's also one of the reasons to be called an Empire.

In summary, it was because the need to legitimate the new government both internal and external, solidify the rule and maintain the cohesion between the states, so no one tried to declare independence from the fledgling empire!

Sources: I mainly used this one article because it's perfect and answer your question precisely, you can check it out and the sources the articles cites for a more in-depth look

Oliveira, Eduardo Romero de. "A idéia de império e a fundação da
monarquia constitucional no Brasil (Portugal-Brasil, 1772-1824)." Tempo 9.18 (2005): 43-63.

I apologize if something does not make sense, but feel free to ask any questions or if something needs to be cleared up, again, I apologize for my english.