@thorondragon  
That could be an in-universe explanation for it but originally Lauren Faust wanted Celestia to be a Queen. She said so on her DeviantArt account and the leaked Pitch Bible even called her a Queen. Hasbro for whatever weird reason axed the idea and told Faust that kids think all queens are evil because of Disney. This doesn’t make sense for multiple reasons.
 
For one, Disney only had two evil queens (the one from Snow White and the Queen of Hearts from Alice in Wonderland) and those two examples were many decades ago. The other queen characters they had since then were good (they were usually minor characters but their later movies made some of them into main characters). Secondly, My Little Pony has had quite a lot of benevolent Queen characters from past generations (e.g. Queen Majesty, Queen Sunsparkle, and Queen Rosedust). So that excuse just doesn’t make sense given that information.
 
I think they made Celestia a princess because princesses are marketable, and that means more toys to be sold. It’s kind of the same reason why they made her toys pink for a while. Even though it didn’t make sense, they thought it would make money.
 
I’m honestly glad they finally made a good Queen character for G4 though. Especially considering she’s both a Seapony and a Hippogriff (I have a soft spot for those creatures). They probably got away with this by giving her a princess daughter but at least we’ve finally got a Queen.

The Not-really jerkass queen lady I always knew I wanted!

QUEEN NOVO > _{^{princess celestia}}

@northern haste  
she is the sole ruler of her people. celestia always intended the throne be shared

It’s also nice that she is actually a Queen and not a princess