Many of those 767's have been repeatedly updated interiors. In fact the 767-400's are in the process of being updated again to the newest interiors as being delivered in the A350's. Once the pax gets in the plane, they don't know the difference between a 767 and 777, especially if the latter has a newer interior.

Delta seems to not like to use any aircraft with excess range, generally if you have a 777 it should be flying on 12+ hr routes. 10-12 hrs goes to A330's and anything less falls to the 757/767...