You play ORAS? If not, don't read the rest.
But if you have no problem with spoilers, in the second charpter of the game Zinia explain all the stories to the player and this include why Kyogre and groundon do primal reversion, and another things about mega evolutions.
She tell that Kyogre and Groundon true form was their primal forms, however to stop them from destroying earth, Rayquaza nerfed them into the normal forms they have now. So in resume, that is why only thoose two can do primal reversion. Because they were nerfed, Rayquaza was not. (But Rayquaza can mega evolve)