My favorite theory is that it's related to zoning, permitting, and the urban built environment. What happened in the early '70s is that many of the US's major cities saw broad downzoning which compounded the effect of urban flight to suburbs, urban renewal, and the construction of freeways in cities which started the decade prior. This ensured that the traditional productivity gains of cities could not be shared as equally any longer. It led to a lower labor share of income (higher share to rent/land) and longer commutes for suburbs which effectively lowered their share of wealth. Productivity gains since have mostly went to increasing economic rents for land owners and monopolistic capital owners. After California's Prop 13 in 1978, many other states followed with their own property tax caps in the late '70s and early '80s which led to even more of the share of economic gains going to property owners.