While I agree that power is the primary issue leakage does have a correlation with "degrading the off" value.

Specifically the effects of drain induced barrier lowering becomes very significant in the sub-threshold region as the channel length is reduced. This has the effect of both increasing drain current (leakage) and making the transistor more difficult to turn off, that is a larger gate bias is required to turn the transistor off.

While "leakage" isn't the cause, you do get leakage and a harder to turn off transistor at the same time, at least for this type of leakage.