IIRC correctly there was some opposition particularly on the right based on the idea the investment money would be under the control of political types who would try to use their leverage to further their socialistic goals.
From EBRI's summary:i
KEY FACTORS TO SUCCESS: Despite initial opposition from labor groups and veto threats from the Reagan administration, Congress ultimately enacted a plan that reduced federal spending and eventually won strong support from federal workers, particularly because of the Thrift Savings Plan (TSP). Lawmakers deliberately and carefully insulated the TSP from political manipulation and minimized the impact of the federal workers’ investments in the financial markets.Now the Trump administration is pushing the TSP board not to include Chinese stocks in the I (international) fund. (Some in Congress are pushing a law forward to effect the same goal.)What it means is a lower return on my money because they view China as an adversary.
I hope all those conservatives who worried about political considerations impacting TSP investment decisions back in 1986 will now oppose this move.