Most likely because Cadence and Twilight were made alicorns, and not naturally born alicorns like Luna and Celestia. Cadence was originally a Pegasus before she was an alicorn. there's a book or something about Cadence's back-story.
Why on earth would you think I'd be angry at you XD. In all honestly i think they changed Luna's design because of the fans being like "Why is Luna's hair not wavy aksfmpaisjf" *shrugs* But Cadence was originally a Pegasus. there's a child's book in which she explains what happened.
Imagine how much rage Hasbro would get from fans if they established that Twilight would become immortal. They'd be forced to make the rest of the mane 6 alicorns, which the fans would rage about even more considering how there would be 9 alicorns. Hasbro just can't win.
I don't think that's Hasbro being unsure as it just being that Hasbro consists of multiple departments and poor communication between them. Some people there, believe it or not, are not knowledgeable about the spelling of Pony names and honestly don't care. They just write Cadence and be done with it. I've seen things like Apple Jack or Twilight Spark. Don't tell me you think Hasbro is unsure about those names too? It's silly to think that Hasbro is unsure about the name of a product they've already released.
Well, everyone is of course free to use whatever name they want. I just hope you won't start calling me Murio or Nerio or something because you think Nurio looks stupid. Just about any source, including the writer of the episodes Cadance appeared in, reports her name written with as Cadance. Her name is as confirmed as all other canon names.