I am just guessing but I think that a global economy has alot to do with it, on the grounds that a global workforce is now at our business's finger tips and this has put pressure on the middle class and incomes the companies know they don't have to pay anymore....
in addition to this, but related to this...ceo salaries have risen and they are taking more of the company's alotment for salaries and compensation for themselves, verses sharing more of it with the workers....this is happening, simply because..... it can happen with a global workforce at their fingertips...
tied in with all of this is our trade treaties that seem to be weighted in the favor of the country we have made the trade agreement with....
and, there are tax laws that put favor in the hands of businesses that do move their businesses over seas, whether just on paper or with actual plants or factories.
Add, the loose as a goose borders and illegal workers from Mexico willing to work for near nothing and ya got a diminishing middle class income standing.
those that are wealthy or those who hold higher positions within a company, have less competition....they are not competing with a global workforce...
therefore they can call for more of the compensation pie....and are getting more of the allotted compensation pie now....
regulation, trade agreements, bailouts, illegal immigration, tax laws, etc... changed over the decades have put the advantage in the very upper income's hands....it's an
indirect redistribution of the country's wealth...or could be considered such, is my understanding of it.