Pokemon is the kind of game that can appeal to virtually any age group. The remakes sell so well because a lot of the little kids that are just getting into Pokemon were either too young or not alive when the originals came out. They played X/Y, their brains exploded, and to them OR/AS are brand new games.

Stockholm.

The changes and innovations made in each new installment.

It's a fantastic adventure rpg franchise rated E for everyone.

It's a formula that's so deep and satisfying that we're usually cool with just having new worlds to explore and new Pokemon to catch. And when it's not that, we want to play new versions of old games we played as kids. They just hit on a game concept that's that good.

VGC sales tend to disagree with you here

Gen 1>2>4>3>5>6 in terms of the main games. So if anything it has slowly declined each generation, the DS being the saviour selling nearly 2:1 vs the GBA allowing gen 4 to take 3rd over gen 3. So its really not even close to being the behemoth it once was even if sales are still really strong.

From a huge pokemon fan i wouldnt say its as strong as it was in the 90s. I think its even better and its still obviously huge, but in the 90s pokemon was bigger.

Either way, its a format that will not get old simply because their is countless possibilities with abilities, pokemon, items, etc. Think of a RPG game like Final Fantasy. In FF you can use how many characters? less than 10. Pokemon has over 700 plus abilities, plus held items. It may be relatively the same format every time, but it continues to grow the possibilities and scenarios

same reason people buy COD without any major change

It's a unique franchise that no other games comes close to replicating. They also know how to market Pokemon. In the anime most Pokemon say their names and in the anime only certain Pokemon of the main characters' evolve. Those are strictly marketing decisions to drill the names of the monsters into your head and to highlight cute (squirtle) and cool (charizard) Pokemon.

CODs made more changes in 5 years than pokemon has in 20.