I think you just answered the question. The interstate highways come out of federal funds and state highways are solely on the back of the state taxpayer. Alaska with its tiny population compared with land area needs all the help it can get. Hawai'i with its small land area and pretty large tax base has less justification for not building all its own roads. Thanks mate!
By the way, Canada does not have interstate highways, it has interprovincial ones... And you can drive from the lower 48 to Alaska by driving from, for example, Seattle to Abbotsford, British Columbia, up to Prince George and on to Dawson Creek which is Mile 0 of the Alaska Highway which will take you to, (wait for it...) Alaska! Set aside about 2 1/2 days for the trip and don't try it in spring or autumn.