That is because the mainline east/west of the BNSF crosses northern Idaho and runs from Seattle and Sumas all the way to Minneapolis and then down to Chicago.
The next east/west line runs through SLC/Denver/Omaha/KC.
The southern east/west line runs from Barstow/Flagstaff/Albq/Dallas/Memphis/Birmingham.
The UP also has several east/west lines.
The line you mentioned handles most of the lumber moving east out of OR, WA, ID heading east.
FYI, what the railroads like the best is moving coal, oil and grain.
That way the WHOLE train starts at one point and delivers to one point. No stopping along the way to pick up another ten cars from this mill or twenty from that mill.
LOTS of coal moving along the Washington side of the Columbia River...a LOT!!!
All that shale oil and coal in Wyoming and nearby areas is flowing through on those trains. The containers moving east are full of Chinese and other Asian goods.