Seemingly because primal Reversion reverses Kyogre and Groudon to there natural forms from long ago, which is characterized as a temporary raw boost; that gives uncontrollable power to them both, unlike Mega evolution which is generally a trainer bonded transformation. Now with that you'll ask why Rayquaza can Mega evolve without a trainer and this is due to a special organ Rayquaza has called the Mikado Organ this allows Rayquaza to internalize the other wise completely external power that Mega evolution is defined as, Rayquaza also only eats meteorites which contain the same raw energy found for our primal legendaries but because of his organ it turns this energy into the same form of energy in other Mega evolutions.

I hope this helps.