system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5. What would make a store not accept certain credit card brands? e.g. Taking visa not Amex, etc?
gpt
Amex has a very high processing fee that it charges to merchants that accept it. Given that most customers already have a Visa/MC in addition to an Amex, why take it? 

Amex is usually about 25% market share, and Discover is about 5% market share. From a merchant point of view, it might not always be worth it, the exception being businesses that cater to business customers or higher end customers, then accepting amex is a greater necessity.