The ROPS height is dictated by an international, not USA, standard and was really established by manufacturers being sued and having the negative publicity of machinery deaths.
"In the United States, standards for ROPS design and utilization for tractors were first developed in 1967 by the American Society for Agricultural and Biological Engineers. ROPS legislation was passed in 1975, with OSHA requiring that all tractors manufactured from 25 October 1976 onwards be equipped with ROPS. In 1985, the development of a new voluntary safety standard by the American Society of Agricultural and Biological Engineers (S318.10) encouraged an initiative by American tractor manufacturers to equip new tractors over 20 horsepower with ROPS."