They probably just wanted to draw the line in the sand that primal devolution is a bad thing m'kay? Despite whatever game bonuses, it seems that the story might be treating it like shadow pokemon, power at the cost of madness. Mega-evolution is power of friendship, so the hero Rayquaza mega-evolves based on positive feelings while the other two go through devolution due to negative ones.
This isn't new, since the very first primal (Primal Dialga) was depicted as a very very bad thing too.
While there isn't any information explicitly ruling out Primal Rayquaza, there also isn't any to support it either. Please stop bringing it up. We got a mega, be happy.