It's a nostalgic game that is easy to keep up with that isn't complicated. Moreso, it teaches and promotes values that society as a whole values such as teamwork. As for me personally, it is something I can play to pass time and is one of very few games where I can play with friends all over the world in that style.
For me, it's partly nostalgia. But honestly? I love Pokemon, even now, and I enjoy the games. And they're always improving the gameplay, so its great to have something familiar, yet something improved. From Yellow to ORAS, it's AMAZING how far we've come! I'm looking forward to whatever comes next.
I think its the combination of nostalgia, but also replayability is key. Every time you play one of those games its a different experience. Different starter pokemon, different ones you catch and choose to level-up. Simple enough for kids at its base, but complex enough to keep the older crowd invested. Though it really is time for the archetypal story to change.
Even though it's repetitive, I'm a long time Pokemon fan. Though I do wish that one day they'll mix it up and do something different to the main storyline, it's that basic formula that got me into the games. So I'll keep enjoying them for now.
People love what they know, therefore the known formula attracts people because they loved the older Pokémon games so much. Furthermore, there are people who collect the Pokémon video games, as well as other Pokémon merchandise. I wonder if the Pokémon collection will ever be worth a great amount of money in the future?
I really don't know. Something about becoming the best pokemon trainer in every single time is somewhat appealing isn't it? lol. I honestly don't know. I'm actually new to Pokemon so I don't have the nostalgia factor that most pokemon players my age have. The only Pokemon memory I had from when I was a kid was Ash Ketchum and Pikachu and his Misty and Brock trio. I only decided to play the pokemon games because a friend of mine (who's actually looking for a job atm and doing his master thesis lol, I used to find it laughable that he could enjoy a children's game) convinced me to try HeartGold. I said what the heck and ran it on an emulator just to see why he likes this game so much. Little by little I learned how the game worked, I learned the strategies required to win, the different types of pokemon, etc... and before I knew it I had already finished FireRed, Platinum, Emerald, and bought a 3DS and Omega Ruby and X and finished them and already excited for the next game. It all happened so fast that I'm having a really hard time finding an answer to this question. Why do I still buy these games? haha.
For me I guess still being interested in all things Pokemon is probably nostalgia, to be honest. I loved the games when I was in my teens and it's something that reminds me of being young and carefree, I guess. I also just think the whole Pokemon franchise is one that keeps reinventing itself, such as with Pokemon Go - so people don't really get bored with the series.