I wanted to chime in, because I made an interesting discovery. Of course, it only concerns two kids I know, but I was surprised myself.
I was at a family friend's house, and they have twin boys, 10 years old. They love Pokemon, so they loved it when I showed them all my Pokemon on the DS games. I even let them play Pokemon White for a bit, and they loved it. Then, I decided to bring over a Gameboy Advance and a copy of Pokemon Sapphire, to see what they thought of an older game. I was kind of nervous that they'd think it was dumb, considering they had just got done playing some of Pokemon White, but they really loved it.
So, then I thought, "what the heck", and after they beat the Elite Four, I brought them a copy of Pokemon Yellow that I used to love. And guess what? It was their favorite! They liked it a lot more than Sapphire and White. They thought the sprites looked cool, too. ^_^
Anyway, I think that kids prefer simpler games, and that since Yellow was the simplest, they were able to get the hang of it quicker. To be honest, even I would've been put off by Pokemon if it had been introduced to me with all the types and complicated features the games have now. I know my 5-year-old self would NOT understand Pokemon abilities, nor would I want to memorize all the types, items, attacks, and creatures you can choose from. It's pretty overwhelming.