>>55563376because volcarona was clearly intended to be a legendary but for some reason was demoted during development.
this is why larvesta is an afterthought mon only available as a one-off egg, why volcarona has such reverent pokedex entries, why volcarona has its own overworld encounter, why the line is so extremely late in the dex (sandwiched between the pseudo and the first legendaries), and why volcarona's BW learnset stretches all the way to 100 upon which it finally learns its signature move which mirrors many other legendaries with signature moves such as reshiram, zekrom, and mewtwo.
why it was demoted to a regular mon i can't say, but all the signs are there.