Honey, they've shrunk the middle class.
Upward mobility first requires gaining a middle class income, on your way to a higher standard of living.
What happened? Government.
Low incomes were exempted from income taxes, then taxes hit hard when you approach middle class. Plus you lose the public assistance benefits awarded to lower incomes.
Welfare reform? Abandoned.
Also, runaway regulations raise prices, making it harder to make ends meet.
Failure to enforce immigration laws increases the ratio of low incomes. First, millions of illegal immigrants arrive poor, although hopeful. Second, they drive down wages, so others find it more difficult to find work.
Government encourages low incomes, but punishes efforts to become self-sufficient.