It's because the wealthy are getting wealthier...
...and the poor getting poorer. The middle class is shrinking, many who were middle class being forced into the ranks of the poor. It's top-heavy. When either end of the wealth spectrum becomes too weighty, it's not going to work for long. It's particularly not going to work for those being squeezed out. That's where revolutions come from. We've got a wealthy class whose attitude is "let them eat cake!"