For some decades (1981 - 2008) Citi was both the owner of Diners Club International -and- the franchise holder in many markets. I think that Citi wrote a contract that allowed the North American franchise holder to migrate to MasterCard network, and BMO inherited that contract when it bought the franchise in 2005. I expect that the contract has a renewal date, and perhaps Diners Club International will be able to negotiate a different arrangement when the time comes, but perhaps Diners Club is receiving an income flow from the MasterCard transactions for doing next to nothing and is satisfied.

As a practical matter, would North American cardholders really want to trade MasterCard network acceptance for a mix of Discover with vestiges of the Diners network?