Iselle and Julio are both Category 1 hurricanes, and they will both potentially impact Hawaii. The good news is that Iselle is expected to weaken as it gets closer, and will most likely hit Hawaii as a tropical storm. Same thing with Julio, plus Julio will pass to the north of the island chain, which means less of an impact for places like Honolulu. So, this doesn't look like a disaster for Hawaii, but it sure is rare! Here's why we don't see many hurricanes hit Hawaii. It's really all about the upper level wind pattern.
As systems form south and east of Hawaii they can move northwest toward the islands. As the storms get closer to Hawaii, the upper level wind pattern changes to westerly or southwesterly typically. This helps to tear apart hurricanes and tropical storms before they can make it. Also, the steering pattern often keep storms south of Hawaii altogether. The upper level winds are a bit more favorable in this instance, allowing for these 2 systems to make a run at Hawaii.
Secondly, the water temperatures get cooler as storms move closer to Hawaii. Not as cold as the west coast, but definitely not prime for the intensification of a hurricane. This doesn't play a huge role, but definitely helps.
The last hurricane to hit Hawaii was Hurricane Iniki in 1992.