Most simply, I think he just doesn't feel the need to run. Previous to Cano's slump, he had three guys behind him that had a great likelihood of driving him in. Cano hit lots of doubles, so he probably didn't see the need to run when he could score on most of those.
Perhaps the biggest reason is the risk- you don't want to take the bat out of Sheffield's or Alex's hands by getting caught. Now this generally hasn't stopped him in the past, so it may not be the case.
As a team, the Yankees really aren't running very much right now, with the only exception being Womack when he plays. Other than his most recent SB, Alex hadn't stole one in awhile.
There's likely something else contributing, but I haven't yet noticed it.