@Piplup;
I'm going to give you the benefit of the doubt and assume you didn't talk to the scientist in Mr. Stone's office after the Delta Episode.
I didn't. I guess I should check this out later then.
Meteorites are not Rayquaza's equivalent of a Mega Stone--they're its food. Its "Mega Stone" is its Mikado Organ--an organ specific to Rayquaza that lets it digest meteorites and store their energy for Mega Evolution. Supposedly all Rayquazas live on space debris and atmospheric moisture.
So that specific meteorite wasn't anything special and it's just another one of many "snacks" for Rayquaza which it uses as energy that can also make it mega evolve? ok then... watching that scene though totally made it seem like the specific meteorite is acting as its mega stone (it even has the mega evolution symbol on it after it transforms in your bag)
Still not exactly explaining why Dragon Ascend is needed then. If it is merely its body that is allowing for mega evo to happen, then actually all Rayquaza should be able to do it regardless of anything.
(Again I still wish they just made it a primal and used the Jade Orb... it'd make sense to me more...)