Clarkson's ego wouldn't allow him to accept that he had been turned down for anything.
Fairly sure I read something with James May where he was saying he thought that because they hadn't been arsed to do a proper show reel for the Americans and had just cobbled bits together from the show with constant references to British culture, was probably the reason they hadn't been offered the gig.
Either way they've had the last laugh, I didn't even make it 10 minutes into the first episode.