Short answer = stock buybacks.
Over the past 10 years, …Wal-Mart has spent more than $65.4 billion on stock buybacks—about 47 percent of its profits. That’s an average of more than $6.5 billion a year in stock buybacks, enough to give each of its 1.4 million U.S. workers a $4,670-a-year raise. It is also, coincidentally, an amount roughly equivalent to the estimated $6.2 billion Wal-Mart costs U.S. taxpayers every year in food stamps, Medicaid, subsidized housing and other public assistance…
The article, by venture capitalist Nick Hanauer, also touches on the fact that most salaried workers no longer get overtime:
you end up working harder—and the harder you work, the fewer employees I need. The fewer employees I need, the higher the unemployment rate. The higher the unemployment rate, the more leverage I have to “encourage” you to “do what it takes” to keep your job. And so you work even more hours, pushing unemployment up and wages down. And that, my friends, is one of the little tricks that keeps you poor and me rich.
Finally, as most of us already know, minimum wage has declined from $11 in 1968 to $7.25, when adjusted for inflation.
http://www.pbs.org/newshour/making-sense/middle-class-cant-g…