Why is Maple so popular?

I think the addiction is also fueled by the fact that there is no reason NOT to play if you've got free time. Unlike games like WoW and stuff where you need to buy playtime, you can just fire maple up at any point, even if it's just for 5 minutes.

Biggest factor is right here.

With WoW, If you're bored one day, you can't just fire it up, you'd have to pay for at least a month. But even if all your NX is gone/expired, you can still fire this up, and it sets back in like a bad rash.