There was just something beautiful about Sinnoh, and really, Gen 4 as a whole. My personal favorite games are Diamond, Explorers of Sky, and Ranger: Shadows of Almia. I stayed up and played those games until 6 in the morning, and even as a kiddo I valued my sleepy-bye time. Here's why:
-- The stories hold up really well. Whoever was on their writing team nailed it. Team Galactic is one of the most moving antags to me, there was something so sinister about them that I was so compelled to go after them. The storyline of EoS is the best one Pokémon has ever put out, you really felt like a hero and if you didn't bawl your eyes out when you die and leave your friend behind, you're lying. The characters were so clear and memorable. And, while I know SoA didn't get nearly as much attention, it remains one of my favorite stories to date with the ideals of harmony and overcoming darkness with light. I was enthralled with being a Top Ranger, and it's one of my most re-played games because I can't get enough of it. Speaking of writing, these games are chock full of LORE. There is so much history to them and I, a history dork, will consume it all.
(It's actually something I missed in XY and SM, there wasn't nearly as much backstuff as the previous games which, I think, is part of the reason they felt so flat. SWSH did better, but I wish there was more.)
-- Despite these games being 10+ years old, the visuals are still really good, even still breathtaking. Spear Pillar and the Distortion World is just... so... I don't even have words for it, that's how much they struck me. Everything in EoS is so perfect for its setting, so rustic and wistful, and it does a great job of taking you to this other, Pokémon world, especially through its use of color and tone. The market place is bright and warm and homey, and the way they juxtapose the future as cold and grey and hostile was so... just, perfect. On the same coin, the styles have just aged really well. They're nostalgic because they have aged so well. The graphics are crisp, but they still manage to be soft. There is still a focus on patterns to get the point across, like the little funky dots on the grass rather than trying to render and texture grass. You don't get that crispness anymore, it's all soft and bubbly. Sinnoh was a spriter's paradise, and the graphics take you back to a simpler time.
-- THE SINNOH UNDERGROUND. PERIOD.
-- Finally, and what I think is most important: SOUNDTRACK. If Pokémon has succeeded with anything through the years, it's their music and these games are no exception. The sound design and OST shine. They're memorable (very important), and they're fitting. It's music that drives the emotion, that drives the breathtaking, and these games absolutely killed it. This was also while we still had that "video game music" style, as BW marked the departure of that style and more "real" music. Pick anywhere in Sinnoh and it'll be beautiful. The themes were so perfect, and they matched up so seamlessly. I can't list all the themes and bits that I adore because I will end up listing the whole region. EoS had a function called the Shaymin Jukebox where you could listen to all the OST pieces you've unlocked, and it was the best thing, and really, DPPt needs it. I listen to Sinnoh all the time. Also, sound design. Lots of iconic sound effects. And let us not forget the PokéChurch in Hearthome city, where the sound just cut. *chef's kiss*
… So I didn't mean to write an ESSAY, but in conclusion, Sinnoh (and Gen 4 as a whole) is nostalgic because it's the last region before a major departure. Gen 5 marked a shift in a lot of technical elements, and it started the leaving of the old video game designs. Its graphics and sound are hallmarks of that time before everything started changing. They are good, solid, well-made games, good enough that its flaws are easily looked past. It's iconic. Am I biased? Hell yeah I am, these are my favorite games of all time. But, if it was just because time and also I was a child, I don't think I would replay them so often as a big kid now.