Because middle class wages have been suppressed to the point where there is no money left over to consume with after paying the mortgage, health insurance, energy costs, etc. Wage increases simply have not kept up with the cost of living in this country. At least not for the middle and lower classes.
Meanwhile, the corporate leadership is paying itself very very richly and demanding more tax breaks for themselves.
So the private sector is in fact doing well for itself, but it is falling on its face in terms of participating in American society.
Add to this the uncomfortable fact that the public sector, while doing its job to support the infrastructure of this country, is a big part of the economy due to the scale of this endeavor. Maintaining roads, bridges, clean water, air and safe food along with waste water management is very expensive and employs a lot of people.
It's time to start talking about how "Supply Side Economics" is a complete failure. Not for the investment class of course, but for American Society overall.
We need to promote "Demand Side Economics". If we don't have what the fancy economists term "ultimate demand", any economy is doomed to failure. It has always been consumption by the middle and lower classes that actually drives the economy. There are hundreds of millions of us. Trying to further saturate the upper economic tiers has already been tried and it is failing!