I'm not entirely sure why she hasn't taken the title of Queen, actually, but I have a theory. She originally intended to co-rule Equestria with her sister, and while a land can only have one King or Queen, there can be as many Princes or Princesses as necessary.

And it would be pretty difficult to find a higher power, given that she raises and lowers the sun every day. [Of course, there is Discord and was Nightmare Moon, but those were special cases and neither should ever be in charge of anything.]