Amex has a slightly higher interchange fee than other networks like Visa and Mastercard. As a result, some merchants opt out of paying the higher fee, resulting in a lower acceptance of American Express cards.
According to the company's 2020 Annual Report, by year's end American Express had:
Maintained approximately 99 percent acceptance among U.S. merchants that accept credit cards
Added 3.7 million merchants in international markets
Received approval to begin operating in China and entered card-issuing agreements with 16 Chinese banks