As a Frenchman, I wonder where you got that perception. Perhaps you were there at the time it was novelty.

If you asked now Frenchmen who had experience with Subway, the majority of opinions would refer to it as « meh » or « pretty shit ». The so-called bread in particular has an especially grim reputation. But the rest of the ingredients also taste mediocre, especially compared to McDo.

McDonald's is such a part of the cultural landscape it might as well be French nowadays. It has much better staying power than any other fast food chain.