Due to the slowed time in dream state, it's still possible that Mal woke up and is working on giving Cobb the kick, but he's perceiving that time as years.
If Cobb is still several levels deep, then those years could have been only a few minutes or even seconds since Mal woke up. She could still be setting up or triggering the kick, or even waiting a few minutes, hours, or even days thinking that Cobb will follow. Maybe she thinks he Cobb needs to figure it out and decide on his own, rather than pull him out by force before he's ready. Perhaps she even harbors resentment that he didn't follow her, that he didn't believe or trust her, and that he's continuing to live without her, so she's chosen to leave him in the dream as punishment.
Going even deeper into it, there's the possibility that kicks don't work. If Cobb is dreaming, then everything we see and are told in the movie could be in the incorrect logic of Cobb's dream. Kicks, dream levels, shared dreaming, all of it could be figments of Cobb's imagination. Mal might not even be real to begin with, she may be just as much a character of the dream as Ariadne and the others.
Or, maybe Mal's just dead.