my understanding is that nw slashed that route in order to free up a spot for nrt-can. there has been a fair amount of discussion on this issue here,... in particular the fact that it's now virtually impossible to get from the west coast to tpe on nw.

i wouldn't be surprised to see it return, especially if nw cancels pdx-nrt, as rumored (on the other hand, others have speculated that that flight would likely be replaced by an extra sea-nrt).