Amex has always been the odd man out with payment processing - it started as a 'credit club', thus the "member since XXXX" printing on the front of the card; you also used to have to pay off the entire balance each month. It's handled differently than Visa or MCard and often has a higher merchant fee. This has meant that shops, restaurants, hotels, and such really prefer that you don't use it as they get charged a higher fee. Amex is also more paranoid about fraud and are quicker to refuse transactions.