We found the same thing when we took our cars to the Le Mans 24 hours race last year. Our five cars (70 'cuda, 71 Camaro, 2 x 56 Chevies and a '30 Model A) all got much more interest than the literally, hundreds of Porches, Ferraris, etc.
I think in France, it was definitely because we all had something different to the norm, but they really loved these old American cars!