Diners may have outsourced transfers to a third party, such as Points International (points.com). It may cost less to make one transfer per month than to have the service provider check for pending transfers more often.
Also, some airlines and hotel chains have become unenthusiastic about large transfers from infrequent customers which are immediately redeemed for flights or nights. One way to allow engaged customers to add to their account balances, but discourage instant redemptions, is to stretch out the time it takes to transfer. I do not know if Aeroplan has taken this approach with other card partners.