I know that Leia was referred to as a princess from the start of A New Hope on account of her being the daughter Bail and Breha Organa - despite the fact that she was adopted. Her royal status undoubtedly stems from position on Aldaraan, rather than any connection to the royal family of Naboo through her birth mother.
Midway through the film, Aldaraan - and, by extension, its monarchy - are obliterated. Yet people still continue referring to Leia as Princess Leia.
HAN: Well, Your Highness, I guess this is it.
LEIA: That's right.
HAN: Don't get all mushy on me. So long, Princess.
(The Empire Strikes Back)
Eventually, of course, she becomes General Organa. But she seems to be known as Princess Leia for much of the Original Trilogy, despite not having a planet to be a princess for.
Why don't people just refer to her as Leia?