Gerald, as Oldfart has correctly pointed out, the FIA brought in the rules to ban the high suspension mounted aerofoils in F1 from the 1969 season, following some failures at high speed. They also banned them in the Can-Am, but their decision to do this came too late to be considered safe for the 1969 season, as many teams had already developed cars with these wings.
Jim Hall appears to be the first to have tried them in Can-Am, and USRRC, in 1966. Its interesting that McLaren didn't fit them to their cars until 1969. Its true, many mounted them to their cars because they saw others doing it. The difference was, some knew why they were doing it, others didn't. Also, some knew what shape the wing itself should be, and how air flowed over and under it, others didn't. I was told once that a racing car wing needs to have a larger face underneath, than that on top, so the air travels faster underneath than it does over the top, to help pull the wing down. I don't know if thats correct or not.
I recall reading a story many years ago about a privateer Porsche in the late 50s or early 60s that had an aerofoil mounted very high above the car, and which was quite effective. Can anyone give more info on this? I think this was considered the first time this was tried. And yes, the theory is that the air is cleaner way up there, away from the turbulent airflow that surrounds the car body.