I think there is just less in-person, physical, self-service card behavior for AmEx to convert. I have also seen AmEx a more useful card for the CNP market (corporate purchases, online invoices, in-app purchases) and concierge-like use (not consumer-oriented/positioned POS). Physical, self-service use of the card (which is usually the only way you see tap done) did not seem to be a common event.

Aside from AmEx's own promotions (encouragement to online or in-app vendor use), I could speculate AmEx shoppers were quicker to move to delivery and other online platform services faster than other networks that still have substantial debit card use which may lean more toward in-person purchases. Those behavior changes would not favor tap/contactless spending specifically.