Many people do not find sweet potatoes palatable and won't eat them. Rice doesn't have this issue. The whole point here is to introduce Vitamin A into the food supply of people with limited options at risk of Vitamin A deficiency without changing the diet so as to encourage adoption.
This is no different than when western countries started iodizing salt. It was much more effective at eliminating iodine deficiency than telling people to just eat seafood.
Or by adding back brown rice to cookers. Same deal as whole grain breads, a healthier option. Won't work for sushi, but sushi comes with fish slices on top too, so...
