According to Dl, the flights were usually quite full (>75%). Rather the reason for dropping them was the $ paid per passenger wasn't high enough.
To explain the higher number of bargain shoppers, look at the routes feeding this service, JFK, LAX, ATL, SFO, YVR, SEA... All of which have multiple direct asian flights for the high paying business traveler to choose from, so naturally why would they choose to have a layover regardless of the airport.
Now, as stated by Delta, PDX local traffic was filling the NRT flight 30%-40% and much of these passengers pay premium fares for the direct flight or biz/first class. Now factor in a carrier willing to code share with AS, and you get feeder flights from places with no direct t-pacific service like Boise, Spokane, Oakland, Eugene, etc... With each of these feeders providing a few biz passangers each, plus travelers from the carriers own hubs (in the CO example you get another t-pac option out of IAH or EWR). I'd bet money the final result would be a plane full of profitable passengers.
One more note, Delta did state "the INS problems did factor into our decision". The INS @ PDX is now going through a major overhaul, with the director resigning. Lets hope the new team learns to loosen up.