For me, it was a matter of acceptance and being too young to see it for what it is at the time. The whole thing is very nonthreatening and seemingly safe, which seems to be the recipe for mainstream popularity. But aside from it being a social thing, it's mostly their biggest marketing scheme. As soon as you collect all the cards, new ones come out. When you catch all the poke'mon in the game, surprise surprise, a new game comes out.
It's mission statement is "Gotta Catch 'Em All" for a reason, because you can never catch 'em all. It's like textbook drug addiction.
Final thought: Am I the only one who finds it weird it's still popular? I mean, you'd think everyone would've wised up to their money schemes by now.