The issue is more with ERISA, which came about in the 70s. It requires all employment-connected retirement plans to act “solely in the interest of participants

The issue is more with ERISA, which came about in the 70s. It requires all employment-connected retirement plans to act “solely in the interest of participants and beneficiaries” for the “exclusive purpose of providing benefits” to them. In that context, the fiduciaries do have an obligation to maximize risk-adjusted returns for the benefit of the plan participants as a whole. While the fiduciaries have latitude in selecting investments, intentionally investing in something they know won't maximize returns to the plan participants would not be allowed. The "benefit" of a below-market-rate affordable housing project doesn't count because those benefits would go to the housing occupants and not all plan participants.