Didn't Craig say something about how power saving mode "pulls levers you don't even know about."
Well anyway, I'm sure at least some of the important "levers" are related to cellular connectivity. Sure iPad has a cellular option, but I would imagine that only a small percentage are cellular models.
Perhaps the mode is less useful on iPad. Perhaps the savings possible were negligible. Or perhaps the feature didn't make the cut for beta 1 (with iPhone being 69% or so of the revenue, it seems to take priority - think iOS 7).