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Regional variants are a Gen VII thing, much like Mega Evolutions are a Gen VI thing. They'll still exist in later games, but only as legacy support rather than being actively supported.
This is how Game Freak operates. They add what amounts to a temporary gimmick so as to not be accused of stagnation. If they kept evolving their existing ideas instead of dropping them for the next flavor of the Gen, you'd get better games but people would complain that it's not new enough. Turns out that stagnation was the best problem Pokémon could have had back in the day because it meant that at least the focus was on polishing up the experience.
