Reason I'm still playing it now, is because Pokemon never really left my life. It's been a franchise which has been with me just about my whole life, and it's just something that I've never grown tired of. I mean, I guess I have certain points where I'm totally obsessed with the franchise, and others where my involvement with the game stems entirely from the fact that I'm a moderator on a forum dedicated to it, but it's just always there. I can't say there was much of a reason to why I originally fell in love with the franchise, other than the fact that my 3 year old self enjoyed the games and the anime and found it to be a fun pass-time, which lead me into the fandom, and had me follow the franchise for years to come. I guess Pokemon was a pretty creative and innovative idea back when the originals saw the light of the world, but I reckon the Pokemon themselves were mainly what brought me into it all, and I reckon that's part of the reason why the franchise grew so big. If it'd been regular animals or bad-ass monsters or something, I honestly doubt it would've affected the size of the fandom, because I find Pokemon to have a certain charm to them, which attracts both younger and older people, explaining why we have so many different people of so many different ages still following the franchise.
All in all, reason I'm still into the franchise is because it's still a great franchise, and because they're still able to bring something new to the table with each new release, keeping it all exciting enough for me to stay. Even if I've been kind of disconnected from the games since the fourth generation due to not owning either a DS or a 3DS, it still manages to keep me interested through just watching playthroughs or simply discussing it here on the forum. I simply like Pokemon because it's a likable concept :p