I think this is entirely a business thing. "Disney Princesses" is a brand, and Frozen is an entirely separate brand under the Disney umbrella because of its popularity and they didn't want to dilute that by lumping either Anna or Elsa in with the other princesses.
That's why when you go to the
Disney store they have Princess dolls, and Frozen dolls. Or Princess pyjamas with a bunch of princesses on them, or Frozen pyjamas. I think there's very few product lines that crossover between the two. Like you don't see pyjamas with Aurora, Repunzel and Elsa together. But you do see Aurora, Repunzel and Tiana.
It's entirely about keeping their cash cow separate and not being over-exposed (yes, I realize that's a funny comment in regards to Frozen) within existing product lines, or diluting their value to licencees (like toy manufacturers) by labelling them just another princess.
Yes, they probably should have had a coronation for them in the parks, but as far as I can tell it's a business thing to keep one brand (Frozen) at a higher level/priority than another (Disney Princesses)