This is the last flight of what was basically a Douglas rocket family design. It will be the end of the Delta family of rockets.
This rocket had high costs due to isogrid tank construction. Boeing also could not initially get a low insurance rate for Delta IV due to the Delta III debacle. But what I think was its main issue was the low construction rate. Without enough orders the fixed costs of running a separate production line for this rocket were never amortized properly.
I think it was a good design for the era but competitive pressure and bad environment did this rocket in.
Delta IV and Atlas V will be replaced with the Vulcan rocket. ULA is supposed to be a merger of Boeing and Lockheed Martin's space business. But it is headed by Tory Bruno who used to work for Lockheed Martin. Boeing's space launch business basically evaporated after Falcon 9 was introduced. So it is Lockheed Martin who is in control of this show I think.