I presume in Alaska's case, at least, it's a funding technicality. As you said, the roads are not built to interstate standards (with a few exceptions around Fairbanks and Anchorage, maybe). For example, without prior knowledge, no one is going to guess that the AlCan is an interstate, as there aren't even any signs indicating such.

I would imagine the (anti-road) weather in alaska might be more extreme (maintenance-heavy) than the continental united states.

Not sure how the hawaiian interstate fares against the environment - could be low-maintenance, could be daily ocean+lava attacks.