I cannot answer the when question, but as far as why goes, I do not think that you have to overthink things here. Eugenics was very popular in the United States and, indeed, much of the Western world up until the end of World War II - indeed, even afterwards in certain corners. You can look up proponents of eugenics and find names ranging from Teddy Roosevelt to George Bernard Shaw to H. G. Wells to even Hellen Keller. It was a concept that was deeply imbedded in the mainstream, and the only prominent institution that I can think of to take a strong stance against it was the Roman Catholic Church.
In short, finding out a prominent individual from the early 20t century supported eugenics would be like finding out that a white Southerner from that era was racist against black people or that a German from the era was antisemitic: it is not to say that all Southerners were racist or that all Germans were antisemitic, of course, but you would be hardly surprised to learn that a particular individual from one of those areas was.
