*How* is pretty well understood. There are clear psychological addiction elements game makers can employ in order to make their game addictive. There's a cookbook for that, too:
Why Candy Crush in particular? Aggressive marketing and a nice heap of luck to get early adoption which drives network effects in making a huge hit.
On a personal note, I really don't get why game producers, even if they do want to produce such a game put that as an outright selling point. I've seen "install this game, it's really addictive" pitch countless times. Why would marketers think that is good? Do people really seek that out?
I want my games to be fun, not crack cocaine. I can recognise addiction inducing elements in the game, and generally don't mind them, but if it's all that a game offers, I'm outta there.