D is connected to C by the underground walkway, so you have to hike down C to get to the main terminal. This was a issue the last years of the hub as you saw a lot of local traffic passengers flights arriving and departing at D, meaning you had to take the underground and C to get to D. Not really a modern airport design this should have been part of the original design. It would not be attractive for even others airlines to use D even if had some kind of agreement from United because of this issue.
The bottom line is you have airport where the main terminal and A and B concourses are 60 years old although has goon through various remodel projects and your newest concourse that opened in 1999 sits vacant.