That share of income from wages started its downward trajectory in 1971, when the US removed the gold standard and money-printing replaced hard money. The result was a man-manipulated interest rate fueled economy. Low interest rates benefit existing asset holders the most since they most definitively increase asset prices. Think about the simplest discounted cashflow, a perpetuity. When you change the discount rate r from 4% to 1%, the perpetuity value increases by 4x.