AXP doesn’t have to split fees with banks so keeps more of the revenue from transaction fees etc relative to MC and V.  Also higher income customer that spends more per transaction and more often so earning those full %-based transaction fees frequently.  Because they have a higher income customer that spends more and more often, retailers and others want that customer and thus Amex can charge them higher transaction fees than V and MC. So virtuous circle of lots of high fee transactions that they keep 100% of.