I believe Alicorns are like a rare genetic mutation so Alicornism is a genetic trait carried through generations but what makes it so rare is that it is not always inherited so it can skip multiple generations so natural born Alicorns are very rare and come once evey few generations though if you're lucky enough a natural born Alicorn could help ascend you to being a Alicorn by replicating the Alicorn gene in the pony who wants to ascend.