There are several reasons. Pricing, it is usually 2K cheaper, even more this past trip in Oct/Nov as JL really discounted the J class seats. Scheduling, the direct flights to HKG that allow same day connection to DAC arrive between 4 and 6 in the morning, providing a very long day hanging out in the CX lounge. They have 4 great lounges at HKG but 12 hours is hard to do. Hotels at NRT are very reasonable and I use that night to get on the new time schedule so I am ready for work in the morning after a 9pm arrival in DAC.