AmEx is seen as the card for the well-off. They usually have stricter qualification requirements and are well-known to never issue another card to someone that

AmEx is seen as the card for the well-off. They usually have stricter qualification requirements and are well-known to never issue another card to someone that discharges their AmEx in a bankruptcy. They have perks and reward programs that are attractive to the people who can qualify. They're seen as more exclusive, so restaurants that are trying to pander specifically to richer people may see it as an opportunity to bolster the impression that only other rich people eat there ("The clientele is so upscale here they don't even have to accept VISA!").