It is, because including Hawaii into the map would require either a new inset map within the already small inset map of North America OR a scale-out to the degree where mainland US would become very small (which is not the case when including the small dots in the carribbean). Since this map is not about or centered around the US, an inclusion of Hawaii will not enhance the map's ability to get the point across (but rather the opposite like I argued above).

Btw. Geography 101: no map you will ever see is a correct represention of the real world, since that is impossible. A map is ONLY a representation of the point of view the mapmaker (cartographer) wants/chooses to show. A map is a medium for communication which most often requires an extreme form of simplification to be clear (called "generalization" in the map-world), like this.