Bevendre

“Why aren’t there more Megas in Sun and Moon? These Z-Moves are super lame!”

Gee, it’s almost like it’s a new mechanic and Pokemon has never focused heavily on a new mechanic outside the generation of its introduction. 

With every generation there is a new professor, one who specializes in that generation’s mechanic.

Oak- Pokemon (1st gen, intro to series)
Elm- Breeding (2nd gen, intro to breeding, gender, and friendship)
Birch- Abilities (3rd gen, Intro of abilities)
Rowan- Evolution (4th gen, physical/special split and controversial additions of new evolutions with new mechanics i.e. evolving in certain areas/knowing certain moves/etc.)
Juniper- Connectivity (5th gen, intro to Game Sync and dream world, both largely abandoned)
Sycamore- Mega Evolution (6th gen, need I even say it)
Kukui- Attacks (7th gen, Z-Moves)

It’s always been a thing.  Chances are that there will be more Megas, and possibly more regional variants in the Sinnoh remakes, or Eclipse version.  Patience.