Mega Kangaskan was the king of gen 6. Then, in Gen 7, it became almost useless with the parental bond nerf. However, Mega Rayquaza, who was much more powerful (Got banned from Ubers) stayed just as OP. why?
My guess is that Kangaskhan was a Pokémon legal for use in VGC and Battle Spot, the most prominent official formats, while Rayquaza could only be used in the Free Battle Spot, which isn't as formal as the others. Kangaskhan became outrageously overused and off-balanced the entire VGC meta, the format officially endorsed by Game Freak and the Pokémon Company for competitions, so it was an urgent matter that needed to be fixed. Rayquaza, on the other hand, only affected the FBS, a format that was always kind of meant to just be a place to mess around in and not be all that competitively balanced.