More detail? Alright, here goes!
For me (and it seems quite a few of you), I suppose it's mostly nostalgia. It was a blast when I was younger, and something that I can play and look back on simpler times. I suppose it's kind of an escape from the day-to-day stuff I have to do (work, mainly). Had I been the age I am now when Pokémon first came out, would I still have played it? To be honest, probably not. With the exceptions of very few games (Pokémon, in particular), I've grown out of video games completely since my 360 RR'd a few years ago.
Additionally, I'd say part of it is my almost compulsive desire to complete things. I can never have just one of a set -- it's either all or nothing. When I started faking -- twelve, thirteen years ago at least -- I wanted to make a card set encompassing all known Pokémon. As more and more have been added, I'm still out to continue what I started. My drive to finish keeps me in the Pokémon online community, and having met some great people, I suppose that's a motivating factor as well.