Well, think of it this way. The two games that did have Mega Evolution had some plot around the mechanic. XY was all about discovering it for the Professor and in ORAS it was about Mega Rayquaza. It's just a theory, but we also haven't seen any info about it yet, which is strange. Usually by now we'd of seen some sort of indication or foreshadow. Instead we get a Ring and Crystal for Z-Moves that mimics the mechanic of Mega Evolution to a more limited and balanced degree.
Plus with the lack of Gyms, is there really any need for Mega Evolution? We don't even know if there's an Elite Four or Battle Frontier/Maison. You could say the Kahunas are the E4, but we fight them after each set of Captains. Sun/Moon feel a lot more story driven like the console and spinoff Pokemon games were.
"some sort of indication or foreshadow" like the Mega Rings seen on Dexio and Sina? If that's not an indicator that we'll see Megas in the game I don't know what is.
Anyway, it's more logical and obvious that they'd show off Z Moves (and Alolan forms to an extent) over Megas; they're cool new features whereas we've known about Megas for 3 years now.
And I don't really get your point about the Gyms since no leaders used Mega Evolutions in their main battles in the storyline (with the one exception of Korrina who was the person to give you the Mega Ring). The only league people to use them were Diantha and Steven who, as being champions, made the most sense having them out of everyone.
Anyway, I'm sticking to my theory of Megas being in the games but just with a lot less importance and being given out in the postgame (or some point in the late main game).