LogFAQs > #941698050

I think it's just the main series games that wanted to push back Mega Evolution in favour of a new gimmick, and they felt they needed to portray Mega Evolution as a bad thing, while also hyping up Z Moves as the new ultimate bond between trainer and Pokemon, since Mega Evolution seemed to be reasonably popular. Z Moves weren't as popular, so Sword and Shield didn't try to portray it as a bad thing to make people not want it back, as far as I'm aware. Mega Evolution is still in other things like the anime, Pokemon Go and Pokemon Masters despite this, so even though we probably won't get any new ones, it's going to stick around in some capacity.

Funny you should mention Korrina as well, because she returned in one of the most recent anime episodes, and this time Mega Lucario didn't lose control and go berserk, so it losing control in the first battle was a case of it being unable to handle the surge of power at the time, rather than Mega Evolution being inherently a bad thing.