Looking at the PRPP, over 78% say they don't offer a retirement savings plan to their employees because it's too expensive; it's too complicated and burdensome; it's uncommon to have such benefits in their sector and they don't know where to start; there's no suitable plan for their business; and the financial risk exposure to their business is too great.
Those are all the major reasons. It looks to me as if the most significant answer by far was those things I've indicated.