Because of the dismantling of the unions. I can not see any other factor being much more different than today, save for perhaps the deregulation of the banks where it seems a lot of wealth is concentrated, wealth which, I do think is a zero sum game as, whatever productivity might increase, it is taken away from some and given to some.
Someone mentioned the technological advances, but technology did not start in the 70s. We had the invention of energy before then, TV, radio, aeroplane, car, etc. So, I think that the period where the employees started sharing less of the gains seems to coincide with the dismantling of the unions, which, on a common sense approach also it makes sense as loosing your bargaining power you would expect would mean getting a worse deal.
Someone mentioned the technological advances, but technology did not start in the 70s. We had the invention of energy before then, TV, radio, aeroplane, car, etc. So, I think that the period where the employees started sharing less of the gains seems to coincide with the dismantling of the unions, which, on a common sense approach also it makes sense as loosing your bargaining power you would expect would mean getting a worse deal.