Rambling
There's also the fact that the first two Gens and 4 have the golden opinion lock due to age + circumstances of old internet and public culture
See if something is well recieved regardless of flaws and the person doesn't explore other media, the opinion of "X old gen is best" will solidify and become fandom mandated. Differing from this opinion will be harshly refuted. This opinion surpasses the fandom to where casual and none fans just blindly accept it. Being "the first" also shields it from criticism cuz "well they just started", even though a GB game having 5-6 years dev is ridiculous
It's why people didn't actually acknowledge Gen 2's attrocious level curve and mon rep till 2021. Gen 1 was a buggy mess, but the bugs are favorably accepted as a fun quirk (meanwhile the core bad game balance and world building is rarely ever mentioned). Gen 4 we have Plat memes nowadays, but opinion of DP sucking until Plat didn't come out as the norm until shortly before BDSP. Even then Plat is still a lot slower than RSE
Internet rumors and dev interviews being loosely telephone rumored also further rose tints things as more intentionally vague. The leaks prove the Unown ruins in GSC were awkwardly unfinished and meant to connect to HoOh, not "intentionally mysterious". Same for killing the Ditto Mew theory, Noting that Moltres in Victory Road is a sloppy error due to replacing it with Mewtwo for Cerulian Cave, and Red being treated as a mute pre Gen 7 when the games' flavor text and alt media (Adventure's, Ash) show otherwise. And the fact Grimer/Koffing ARE meant to be impacts of urbanization as stated in the 1996 dev pokedex book, but are awkwardly in abandoned old areas away from pollution is silly
Of course one has to be aware of gaming standards at the time, and Pokemon being a first for actually having a unique monster roster over 50 is nuts. But I feel it gets a bye for too many things that straight up was no longer an industry issue (Gen 1's overworld straight up lagging)
It's an issue that plagues many longtime fandoms, and early Gen elitism definitely shaped aspects of romhacking and fanfics. I've seen it firsthand for the former, not just for Pokemon...
You'll notice I skipped Gen 3, Pokemania dying mid Gen 2 + being unable to transfer mons + aging demographic leaving + Team Rocket no longer mattering heavily impacted the Gen's perception. Gen 3 wasn't really golden standard till around ORAS hype, and then it deflates to neutrality after. Gen 4's anime shift to Cartoon Network actually helped massively to reintroduce casual audiences, and a true new gen of fans came, with Gen 2 remakes attracting Genwunners/Twoers. Even FRLG, as hyped up back then, was a small note with most not caring until LGPE cutified Gen 1, and forced many to realize the sparse postgame. Only romhackers really cared to use it as a template, purposefully ignoring RS cuz Kanto nostalgia was priority, and later Emerald cuz one was stupid to not update Romtool compatibility for addresses, which everyone blindly accepted given bias
Nowadays, you have people still salty over BW. It is the end of "Golden protecting", despite devout Gen 5 fans now. Casually many will still remember the hate of back then, and none fans only care for 90s Pokemania (Kantooo). XY is skipped largely cuz of Gen 5's failure, over relying on Kanto and other Gen Megas rep over its own mons, so people come back and are more disappointed compared to ORAS despite nice online features. But more importantly, Gen 5 marked when Pokemon was no longer as splashable "can interpret anything" for chars. The game's plot and intro was marketed heavily, unlike earlier Gens
Which I feel is the root of this. Early Gens not being as hard defined for certain aspects meant that one can headcanon anything, without consequence due to how vague the source is. Later Gens are less and less vague even for mon personalities, so this might encroach on personal freedom. Similarly it means issues if something contradicts said characterization (USUM's rewrites)
But above all, Gen 8 shook up people from being in denial that GF is seriously behind the times for tech. They don't acknowledge that it applies to earlier Gens as well (Gen 1's absurdly long dev, Gen 2 severely relying on HAL/Iwata so the game didn't lag, Gen 3 just being super basic compared to 16bit RPGs, Gen 4 refusing none tiled design). They dismiss the sprite oddities/mistakes and warp it as "soul" to hurt later perceptions of 3D models (which when Gen 6 came out no one cared besides color). It's very forced thinking to ensure older gens aren't percieved poorly, as the internet since Gen 3 mandated the "old gens better" opinion
It's toxic, and unfortunately likely won't stop
TLDR: Gen 6 came out right after controversy + poorly repped its own mons in favor of Early Gen promos (Kanto starters gift, Megas, ORAS), so you won't see much defend it compared to 5/7