Seattle is the terminus of major rail lines stretching all the way to Minneapolis and Chicago. Portland, for that matter, is at the end of the Union Pacific's northwestern transcontinental route. Building expensive port facilities to handle business lost to the Port of Los Angeles because of rare strikes seems the height of folly. Strikes which close down Los Angeles once in a while don't seem to justify the huge capital expenditures needed to build commercial port facilities, particularly when they'll sit idle if there's no L.A. strike, which is most of the time. If you think geography, namely the great circle routes, doesn't play heavily in the minds of ship owners, ask them.
Time is of the essence in the operation of that business.
However, you don't see either the Union Pacific or the Burlington Santa Fe rushing off to restore it. By the way, I use Interstate 8 a lot and can compare its use by 18-wheelers with Interstate 10 and other transcontinental highways. There's no real comparison.