I think it's almost entirely this. At a certain point (around early college age, 20ish), most people I know get a severe nostalgia trip. For me it was stuff like Beast Wars, the original Pokémon games, etc. Along that same vein, most people I know (including myself) stopped after Gold/Silver. The idea of "catch 'em all" had become laughable, it was on a new system that not everyone had, and nothing recognizable was available early. Combine that with how we were at that time 13 or so, and started trying super hard to be "cool" with the age group 3-5 years above us (which Pokémon wasn't) and it's easy to see why those games weren't very liked. It wasn't until X/Y that I even picked up a "new" Pokémon game (although part of that was lack of system), and no one I knew even considered it until Black/White, when we were old enough to not care about appearing cool.
That's how I was. I shunned Pokémon around the time I entered middle school (which was, conveniently, in 2002, the same year RS came out), and started getting into The Sims more. It was only about a decade later, when I was in my early 20s, that I started coming back, and even then, it was pretty much limited to RBY/GSC, aka. the ones I grew up with. None of the later Gens ever really caught my eye as much as the first two, and that probably won't change anytime soon.
Also, if any of them actually went back and played the first games for example, it would be extremely underwhelming. No nature, no ability, no alternate battle types, no eggs, no holding items, objectively awful (although I love them) sprites, Psychic superiority... should I go on? I can see why they wouldn't be fans of it, because it wasn't super-new groundbreaking stuff for them like it was for us.
Or, more likely, they played FRLG shortly after RS and were greatly underwhelmed by all of the missing features the games had in comparison (yes, even to
Ruby and Sapphire). And, if they first played FRLG after Emerald or especially, DP, well... I can see why they think Kanto is a "boring," "sucky" region. HGSS might have helped things a little bit, but not by a whole lot.
It's sad that these fans are so repelled by Kanto (especially its Gen 1 incarnation), because they're probably the ones who could benefit the most from a new remake. RBY are good games, but they're extremely outdated, while FRLG are both (less) outdated *and* bad. A brand new, fully-featured Gen 1 with all of the latest bells and whistles is what it would take for them to, at least, appreciate Kanto. (Like how many gained a new appreciation for Hoenn from ORAS.)
overall i still put strong value to gen 2 more so than gen 1. its entertaining and still provides a lot of fun for me...to me i just tend to get overwhelmed with the latest games...their are features i just don't want to get into. gen 5 especially felt unnecessarily excess in features.
I feel you on that. I'll still always prefer the simple, easy-to-understand stat system of RBY/GSC to the (over) complicated Natures/EVs mess of later Gens.