While Delta Air Lines may love the Boeing 757 (it operates the largest fleet in the world), the same can’t be said of the Boeing 787. 
Delta Air Lines’ fleet of large widebody aircraft is now made up of Airbus A330 and Airbus A350-900 (with Airbus A350-1000s on order). The Boeing 787 and Airbus 350 dominate the world’s longest routes and it seems that Delta has decided that the A350 is the ultra-long-haul aircraft of choice for them.