I believe DeputyDan is onto the real reason. The Hawaiian Islands are pretty exposed in the Pacific, and pretty much all the diving is on the leeward side of each island (we have steady NE winds most days). Traveling between the islands can be pretty uncomfortable if you don't have sea legs.
The Kona Agressor gets to a lot of sites that day trips boats don't down near the south point of the big island, but they do stay on the big island.
However, if you ever do get a chance to participate in any island crossings, it can be an amazing experience. As much as for the adventure of the trip as for the diving itself. My first diving job in Hawaii was with the no longer operating Free Spirit Sailing club. I was the divemaster/crew. We always departed from Oahu, but we made trips to the big island -diving Molokini and the north shore of Molokai on the way back, trips to the backside of Lanai, trips to the north shore of Molokai. I was on the water just about every day back then, and I took 2 dramamine for the upwind crossings. I never passed up a chance to go though, and still look at those trips and some of the best diving/sailing/sighting seeing trips I ever took.