To get to Alaska from the lower 48 the options were boat or plane, there was no road.
The US tried to talk Canada into a joint venture to build a road in the 1920s  but they weren't interested and saw no benefit to Canadian citizens.
We tried again in the 1930s when Japan was saber rattling in the Pacific. Again Canada didn't want to spend the money.
I would guess the Canadians were leery of letting the American have any interest in the Canadian hinterlands.
Then the shit hit the fan, the big one, WWII, and Canada was afraid they would be viewed as a US ally rather than neutral if the Japs invaded North America.
The Japs has already invaded a couple of the Aleutians so the US persuaded Canada to sign a treaty to let us build the road at our expense and everything on Canadian soil would become Canadian property 6 months after the end of the war.
The purpose of the road was twofold, first a land link to Alaska and second create emergency landing strips from the straight sections as this was the route all those planes being shipped to Russia took.
10,000 GIs built the 1700 mile road in 8 months through some tough terrain over mountains and swamps.
Canada took possession of most of it 6 months after the war ended as promised.


I've driven it 3 times, 6 if you count each way. LoL  Sure is pretty.