Nostalgia- A big factor for me. I was totally obsessed with Pokemon phenomenon back when it first took off in the 90's. I had so much fun with Pokemon back then. Now we're closing in on 20 years later and I am still a little obsessed with the series .
The games - They continue to evolve and improve over time while maintaining the same core game play. I've dumped thousands of hours into the Pokemon series, yet every new installment manages to suck me in and keep me playing for hundreds of hours.
The Fanbase - Usually a demerit for most games, but I actually really like the Pokemon fanbase. Maybe it's just me, but I don't see a lot of hostility, trash talking or any of that foolishness. Pokemon fans actually seem to have respect for one another. I also really enjoy the constant flood of fan generated content. Fanfics (love dem creepypastas), art, music ect
The Anime - I know there are a lot of people out there who find the Anime cringe-worthy, but not me. I'm not sure why, but I still enjoy it. Once I start a series, I feel compelled to see it through to the end..(which reminds me I still have to read the new chapters of Bleach and Naruto)
The Cards - I collected them religiously in my youth, then over the years I moved on to yu-gi-oh, then Magic, then for a long time, nothing. I have recently started playing the Pokemon TCG online and gotten back into buying/collecting the real cards again. Love it so much.