Being popular doesn't always necessarily equate to a second season. Look at MADHOUSE Inc., for example. They are notorious for pumping out only 1 season/cour of anime of popular works, despite the BD/DVD sales being high and the source material selling well.
That said, production for a second season was greenlit in 2014. So I guess fans could say it's about time the ball started rolling.
I, myself, am not an avid fan of the series. Buttttt, I know how large the following of AoT is, so I just thought I'd share. :3