This makes labor less valuable, and decreases labour's negotiating power, preventing an increase in wage from an increase in productivity.
That is, if your work is augmented by a machine, then it's a hell of a lot easier to replace you.
Everyone so quickly forgets, "Let them eat cake."
These will only be exasperated further by the onset of automation and robotics in the coming generation(s).
I don't have a doubt that we could easily turn around the benefits of automation and robotics towards favouring the middle and working classes.
My concern though, is that the few that are currently gaining wealth from reduced labour costs in favour of productivity have a stronger grasp on where this wealth goes, than the majority outside of socialist strongholds like Scandinavia and some European states. And that trend will continue.
Edit: grammar, shit pre\post-tense structuring.
I think the most likely culprit is what Steve Keen says it is: debt.
TL;DR Productivity is kept by corporations, wages are stagnant because of it, and the US doesn't have a strong labor political party.
Raise the minimum wage and index hours worked per week to productivity. Productivity goes up? People work less but make the same amount of income.