Macaroni salad is not a traditional hawaiian food, but my guess (as a chef/restauranteur) is that the reason is simple. Mac salad is cheap. It's also a good alternative to the pig for guests who follow a kosher or halal diet. The presence of tuna in many macaroni salads gives it a bit of legitimacy on a polynesian menu as well.
Likewise, a lot of luau hosts now offer chicken teriyaki, other fish and sushi to appeal to the variety of guests.
In a strictly traditional luau, the menu would be hog, salmon and chicken, prepared hawaiian style, but in such a strict luau, the guests would all be locals who are used to that kind of diet. As hawaii has grown more tourism focused, and luaus have cropped up in other places, they need to cater to a wider variety of guests.
