Huh? For consumers and merchants, Amex and Diners are exactly the same, over engineered franken-products that are just payments mechanism with an unsecured debt facility hanging off the back of it with even more insane fees than Visa and MC charge.
Thats completely untrue.
As closed payment networks AMEX and Diners are able to offer services that Visa and Mastercard cannot. These are mainly relevant for back office functions such as finance and accounting.
By way of example, Diners Club is the Australian Government's card for airfares, car hire and accomodation.
https://www.finance.gov.au/procurement/travel-and-related-services/travel-suppliers.html
The reason for this is the enhanced data that Diners Club is able to provide to the government. There is also built in GST reporting which is ATO compliant which is very relevant for the Government spend as GST needs to be adjusted. As a result of this compliance, there is no need for receipts to be kept. AMEX corporate programs offer similar reporting and the ability to sign off card spend electronically and eliminate paper.
Diners Club does not have a credit facility.
As far as "Insane fees" that Visa and MasterCard charge, keep in mind that the issuing Banks are levying these charges, not Visa and MasterCard. The ancillary fees charged are in direct relationship to the benefits offered. It's only insane if you pay these fees and get nothing in return.
Diners was the original Travel and entertainment card and AMEX was a close second. The reason for these cards in the first place was so that the finance department would only have to pay 1 bill at the end of the month and get some control of their T and E expenditure.
As the market moved away from the core functions of these products and demand for more bells and whistles for card holders emerged, card products changed and we have ended up where we are now - the land of the franken card product.
Cardholders demand more rewards and lower fees. Merchants are seeing increasing demand for card acceptance and want that at a lower cost. Something has to give and we are seeing that now with regulated interchange, reduced points earn and higher fees.