People keep bringing up X/Y as part of the problem, but honestly, an X/Y-like distribution wouldn't be so bad at all. A plurality of Megas were from Gen I, but there was still a decent distribution across the other generations (besides Gen V, but I think that's because at the time they had just had three years in the spotlight). Like, I don't think people would mind so much if it wasn't Gen I Pokemon getting these forms now to the near-absolute exclusion of everything else.
Just for the sake of numbers and only counting the unique pokemon that got megas (so ignoring the fact mewtwo and charizard got two)
In x/y megas were broken down as follows
10/26 gen 1 (38%)
5/26 gen 2 (19%)
8/26 gen 3 (31%)
3/26 gen 4 (12%)
Then with the megas added in oras
13/46 gen 1 (28%) decrease of 10%
6/46 gen 2 (13%) decrease of 6%
20/46 gen 3 (44%) increase of 13%
5/46 gen 4 (11%) decrease of 1%
1/46 gen 5 (2%) increase of 2%
1/46 gen 6 (2%) increase of 2%
Not the worlds greatest distribution, but gen 1 feels like it got an appropriate amount of megas. If anything you probably should take 4-6 megas from gen 3 and give them to gens 5/6 and it'd be an ideal balance. Yes I understand the above doesn't include both forms of charizard or mewtwo (would slightly bump gen 1 up to 31% of all mega forms fwiw)
Heck if anything the fact a gen 3 pokemon got a galar form and a unique evolution shouldn't be surprising since if the mega % is anything to go off of it's the other classic generation most likely to get new forms.