United have scrapped plans to introduce a 787 route from IAH to AKL. They will instead use this aircraft on a new DEN-NRT route it appears.
It appears the most logical reason from sifting through a few articles is that CO bought this 787 before the merger with UA and wanted to make use of their IAH hub for Pacific flights. But since the merger UA announced this week they are shrinking their IAH hub. Pacific flights would remain focused through their LAX and SFO hubs where there are many connections.
This USA Today article is one source of reference, with links to many other articles and blogs also.