I have a headcanon as to why Sunny only stays as an Alicorn temporary compared to folks like Twilight or Opaline. It's because of when she became an alicorn.

You see, when Sunny became an alicorn, the unity crystals were just activated for the first time and magic just returned to Equestria. I feel like, just like how it took time for earth pony magic to be unlocked and dragons to return, it'll take time before Sunny will transform into a true alicorn. The only reason Opaline is the way she is is because I believe she was an alicorn prior to magic's disappearance. I imagine the same kind of effect that Sunny has could happen to anyone in the G4 era as well- if an alicorn transformation was interrupted part-way through. There simply wasn't enough magic yet to fully transform Sunny, and I think it'll take a long time until she's able to complete her transformation.

But hey, that's just a theory, an MLP theory. Thanks for reading.