If there's one thing I've noticed over the years of this franchise, some people are VERY devoted to their favourite mons, to the point they'll roll through multiple games using old favourites whenever possible.
Now while I think that's a pretty unexciting way to approach new generations, each to their own I guess.
But Black and White? its decision to make its main game entirely focused on the unova dex with old mon only in the post game, really rankled these kind of players.
This grumbling then extended to the new pokemon in the crosshairs, which are a batch I've always considered to be mostly great with some real stinkers bookending its front (early game pokemon) and back (legendaries).
Having to replace plenty of recurring series tropes for both fitting standard environments and teaching new players some of the old evolution methods etc led to a lot of "like Gen 1 but not" pokemon which could be seen as redundant.
There were similar sentiments I remember from lapsed players I knew who hadn't played since Gen 1/2.
There was also some stink about linearity because the world map doesn't mask the straight path through to the league at all with its pokeball shape, in actuality it's pretty much business as usual and has plenty of optional side routes and locations with worthwhile exploration rewards.
Oh but I'm getting ahead of myself here, as you may have guessed from my tone, I thought these games were great at the time and felt that the blowback was full of exaggerations and a fear of any sort of shake up to the status quo despite the games being more of the usual iteration.
These entries did so much right in my eyes, the variety and spread of pokemon types was spot on once you crawled out of the slow start, each route or cave tended to have something new to catch right up until the endgame, they even tried harder with the story and NPCs.
Anyway, history vindicated me, Gen 6 arrived and then the people realised, you don't know what ya got until it's gone.
No point @ing me because I'm going to bed, Gen 5 rulez nerds and the poopy early reaction to it led Game Freak into a pit of nostalgia it can no longer escape from.
but it's okay, trubbish still loves you, even if you shunned him