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As many others have said, its nostalgia.
Don't get me wrong - Gen 1 has a certain simplicity to it, from its era of gaming really. All the added stuff the series (mega evolution, new pokemon, contests etc) is controversial for some, and distracts from simple goals outlined in the first game.
And its that simplicity, combined with the nostalgia that makes the gen feel perfect. But its not sadly, gen 1 had a LOT of flaws.