I assume since the flight flew all the way back to NRT it was not a true life or death situation. In those cases they will land anywhere and figure our the logistics from there.

NRT probably had a spare crew as well. Where ANC most certainly doesn't. Plus depending on the time there may not have been any customs staffed at ANC. (quick google search says customs goes home at 4:30)

So while increasing flight time, going back to NRT probably greatly reduced your delay....Glad everyone is safe and that you didn't end up in Goose Bay for a day and a half!