I think this make sense.

She is the first alicorn who born as alicorn.

I think Celestia and Luna and Cadence and Twilight have something in common. It is leadership, responsibility, and modesty.

They proved these virtue. So the Harmony turned them into alicorn princess.

But about Flurry Heart? She doesn't have to prove it!

I think Cadence and Shining Armer must take care to teach Flurry Heart.