The short version is AmEx stopped acquiring transactions directly from merchants in some Caribbean countries and instead shifted to a model where the merchant’s bank needs to act as the intermediary. This is how Visa and Mastercard work. But not all merchant banks decided to work with AmEx. So if a merchant used to process directly with AmEx but the bank they process other cards with doesn’t accept AmEx, then that merchant cannot accept AmEx anymore (without switching banks).

https://www.americanexpress.com/content/dam/amex/idc/assets/pdfs/caribbean-global-accounts.pdf