In my opinion, it's primarily a matter of nostalgia, but *not* in the form of "Oh, people grew up with it!" Rather, it's that Pokemon, as a franchise, fragmented especially hard with Black and White, where gens 1-4 all "built up" game design from the original framework of Red and Blue, then gen 5 threw quite a chunk of that design in the trash to be near-totally its own game, and gen 6 solidified a new direction.
It's not so simple as "Oh, they started kiddifying everything after gen 4", it's that they'd actually been drifting away since gen 3 but held on to enough "legacy code", as it were, to stay very clearly the same franchise. But then Black and White happened. Then Game Freak made a clear break between "old" generations and "new", with a region that was *obviously* a straight-up reboot.
Plenty of people who grew up with gens 1-3 love gen 4 because it contains the culmination of the franchise as they knew it. The entire reason the Pokemon community so frequently comes to genwars is because Game Freak *does not actually make the games as a coherent franchise*. Little about the design process ever asks "Does this gel with the previous titles?". It's "Does this, in isolation, look like a coherent game?"
That's why one can so easily identify Pokemon by generation, even as a detached observer. Because each generation has a different artstyle. Until BW, this was a mild iterative thing. But BW was designed with an *entire* new Pokedex, a region that worked with *no* old Pokemon. And in doing that, they put this habit on their sleeves. They *asked* people to compare Black and White with Red and Blue, and it was obvious they were not the same thing.
TL;DR, Game Freak never actually gave a fuck about franchise consistency over the years, and gen 5 is where the facade of this finally broke.
