Basically what everyone has already said.
For me its probably because the concept is the same that I play it. As much as I welcome an improved version of pokemon, or one
with a darker story, I know it won't be pokemon if the concept changes drastically.
Gamefreak has been making changes to the game, so despite it being the same, each and every game is different in their own way.
And also, building my team of cute little pokemon and training them, seeing them grow to become badass..it's indescribable.
Then, there's also the rival system and the elite four that never gets old
And I'm not denying that nostalgia wouldn't be a factor either.
Plus the nostalgia factor is big and it's become interesting to see exactly how many Pokemon they can come up with.
This.
Although there's always freaky looking ones (like that ice-cream pokemon) but for every freaky looking pokemon, there's 10 more good looking ones.