United offers wonderful discounts on its corporate travel agreements. That is why they can operate as the greyhound of the skies. My employer pushes its US based employees to fly United. I pay for my own TPACs and it is an anyone but United. I note that Delta the elegant lady of the skies has maintained competitive pricing for east coast originating TPACs, and I pick Delta over United whenever I can, so I am biased.
I also find that US nationals prefer US flag carriers.I see it in the premium cabin reflected in the pax mix. In talking with colleagues, when I ask them why they didn't use superior airlines like EVA or ANA or JAL, I get a blank look or some cringe worthy responses. If it was just pricing, they would be flying on the awful carriers like Air China and China Eastern, which can often be less expensive than United. (And yes, I too would fly United ex USA rather than those carriers.)