I don’t understand the appeal of Rosa or Hilda at all
Gen 5 nostalgia, plus in Hilda's case, I know a lot of people absolutely
love the idea of an older protagonist.
But I think most of it comes down to nostalgia. B/W came out, what? 9, 10 years ago? And D/P came out like 12-13 years ago. We're at the point now where a not insignificant percentage of the fandom probably missed out on the first three generations due to age and started out with the DS games. To them, Gens IV and V and all associated characters are their nostalgia. I think that's also part of the reason why so many are apathetic toward Kanto.
Speaking of which...
I don't think this opinion is that unpopular, but I don't mind this "Kanto shilling" as long as the Pokemon getting attention are more underrated Pokemon that actually need it. You know, stuff like Farfetch'd and Ponyta and Butterfree that are basically considered useless and forgettable otherwise. Hell, that might even be part of the reason
why Kanto gets so much attention. A lot of Kanto Pokemon are very simple and bland in design, not to mention competitively outclassed by the newer Pokemon, so why bother ever using them at all otherwise unless you
really like them? And I'll be honest? Outside of stuff like Dunsparce, Delibird, Chatot, Qwilfish, and maybe a few others I'm not thinking of atm, I can't think of that many non-Kanto, non-recent Pokemon that are in that desperate need for an upgrade. But then, I don't play competitive, so what do I know?
Also want to mention that more Johto Pokemon than Kanto received new evolutions in Gen IV (and Hoenn was left out, either). There are more Mega Evolutions for Hoenn Pokemon than Kanto (and Johto and Sinnoh weren't completely left out, either, while Unova and Kalos were probably considered too new to receive Megas). Z-Moves were mostly a thing for the Alolan Pokemon anyway. And there are currently more Unova Pokemon in the Galar Dex than Kanto. This idea many have that GF/TPCI/whatever doesn't care about the other regions is utter nonsense.
The only time when the Kanto focus
does annoy me is when it's the same Pokemon getting the shiny new toys every single time, and I think that is the
real problem that has soured people's opinions of Kanto. It always seems to be Pokemon like Pikachu, Eevee + Eeveelutions, Meowth, the starters (especially Charizard), Mewtwo and Mew, Snorlax, Gengar, etc. that get all the attention because they're mascots or fan favorites. I think if GF/TPCI just backed off from those particular Pokemon, people wouldn't be as frustrated with Kanto. But we all know they won't, and I understand why they won't.