There is only 185k people living on the "big island". I'm not sure why people would expect something other than a rural America level of services, when it is really just a small city of people effectively in the middle of no where.
Note that smaller cities in rural America service a much larger population than just the city itself. The island does not have a surrounding population that can drive an hour in once a week to make use of a big box stores.
Google maps says there is a Walmart, Kmart, Sears and a number of other well known American brands. It's not like they are being blocked from entering the market.
Of course we ended up at Walmart because the people I was with are suckers. Lovable suckers, but all-day suckers nonetheless.