You forgot about Z-Moves...which is fair enough. Z-Moves are the gimmick I probably used the least in any of these games. Taking up a item slot is what killed it.
But, yeah, someone at Gamefreak really doesn't like Mega Evolution. Some of the Pokedex entries in ORAS and S/M really wanted to play it up like Mega Evolution was steroids, ruining Pokemon's lives. I liked some of those designs. And it was clearly the most useful, since it lasted the whole battle.
It probably comes down to a few things:
1. Nintendo wants each game to have a "gimmick". Or rather...I think Nintendo likes each one of their games to be instantly distinct from one another. I think Nintendo in general just likes for people to be able to tell a game apart just by a screenshot or two alone. Thus...why so many of their games feel like they add gimmicks to them.
2. Gamefreak wants to cut back on how many models they use. They've been cutting back on the number of Pokemon they include. So, it just makes sense to "simplify" any new super-forms into a filter over the models they're already using. (Now...whether or not Terasterilized pokemon all just use the same "filter" or if Gamefreak programmed completely seperate models for EVERY SINGLE TERA-POKEMON...I have no idea. Gamefreak is notoriously bad at optimization. It could be either)
3. Trying to maintain balance in the competitive scene. Mega Evolutions were notoriously overpowered. It makes sense to chuck them if they wanted better balance. But...really they COULD have just nerfed them for competitions, like not allowing Legendaries. They were fun in the story. But...that's probably more work than Gamefreak's willing to do.
As for Terastallization....I wish it had a different name, it's a mouthful. And the hats are too much. Gameplay-wise, it's...potentially interesting? But, I haven't really played around with it enough to see all the tricks, yet. Honestly, I don't see a lot of advantage to using a Tera-switched type Pokemon over...just using a different Pokemon with a movepool that matches the type. But, it's probably just to spice up the competitive scene a little, so *shrug*. Tera-type changes aren't really that useful in the main game, from what I've seen.
I could see little crystal figurines being a hit, though. Toys of these Tera-Pokemon could be really pretty, looking like crystals. I could see some people getting hooked on collecting little crystal figurines of their favorite Pokemon with exclusive/rare hats.