” Foreign drivers have run Indy almost since the beginning;”
Yes, I am aware of that. However, if you look at history, in the 60s there was an influx of foreigners.
Yup. Quite a few of them came in because Indy paid a lot of money; when Jim Clark took second in the 1963 race he earned more from that one race than he had earned in his entire Formula One career up to that point (and by this time he’d already won his first World Driving Championship).
And it seems that the money was just better in the US at that time, period. For most of the years that the USGP was held at Watkins Glen, it offered the highest purse on the F1 schedule. This was all prior to the original Concorde Agreement.