It has to do with a combo of general airline industry practices + JV practices. In fairness, at AA and UA, agents will similarly pushback on using BA/AY or LH/OS coded flights wherever AA or UA coded segments exist.
Under airline industry rules, the carrier marketing the first overwater segment on an international itinerary should issue the ticket. Therefore, if a ticket is issued on Delta (006-) stock, the first overwater segment should carry a DL prefix. Defaulting itineraries to DL-marketed flights whenever possible helps ensures compliance.
Additionally, airline Joint Venture agreements weigh the marketing code of a covered flight as a factor in allocating revenue. This means it is slightly advantageous (miniscule on any one ticket, but adds up in the aggregate) for Delta to sell a flight under the DL code versus the code of KL, AF, or anyone else.