Originally Posted by Steve Wilkinson
On the link provided by Vitesse it states "Entrants determined by special qualification session during fall Michigan race."
Maybe some of the other top line teams didn't fancy a trip over the pond? Anyway what we got was entertainment and I still remember Gordon Johncock in the Wildcat, Salt Walther in the McLaren, Spike Gelhausen in the Eagle and the rest.
I think they were decided by a qualification session (at Trenton) in July, because IIRC they announced the 16 qualifiers over the PA at the British Grand Prix meeting in July. Might be wrong but that's how I remember it - I guess they'd have needed to do it a couple of months in advance because maybe it was the first time some of the Americans ever needed to get a passport!
If any of the leading runners didn't make it, it would most likely have been because of problems in that qualifying session rather than not wanting to go to the UK.