Many moons ago, I used to work weight and balance for NH, when they still flew B744’s on this route. FOR the most part this has been their preferred route between IAD-NRT. The ONLY thing I can think that seperates the 2 routes is the winds aloft, and the space of the flights crossing that airspace. Keep in mind, that the JL, NH flights from JFK also depart around this time, and it could be space on the routes with those birds.
Part of this equation is the work required to get approval for polar routings. There is a very detailed set of requirements for airlines wanting to use these routes.