Because President Polk wimped out of his campaign promise of "Fifty-Four Forty or Fight!" Had he gotten us 54° 40' as the northern border of Washington as promised, British Columbia would be ours and we would have continuous U.S. territory all the way to Alaska (the southern border of Alaska is 54° 40'). They would have probably built a road through there in WWII.
A road was built in WWII, (as you well know) but it was inland, over much eaiser country, and out of the range of any Japanese gunboats. Trying to build up the coast would have been much more difficult.