Boeing was very confident Delta was going to order the 787 and felt it was obviously a better product than the A330-900. United would however not make space for Delta on the schedule to get the aircraft in the time frame requested. They instead offered to deliver 777-300's on a lease until the 787-900's would show up. Delta requested 777-200LR's instead as well as additional 767-400's. Boeing said no to both stating they would not build additional passenger 767's and would no longer build the 777-200. Delta went with Airbus.
United also had discussions about additional 767's and Boeing said no as well in the 2016 time frame.