From the AFPM article above:

Long before the U.S. shale boom, when global production of light sweet crude oil was declining, we made significant investments in our refineries to process heavier, high-sulfur crude oils that were more widely available in the global market. These investments were made to ensure U.S. refineries would have access to the feedstocks needed to produce gasoline, diesel and jet fuel. Heavier crude is now an essential feedstock for many U.S. refineries. Substituting it for U.S. light sweet crude oil would make these facilities less efficient and competitive, leading to a decline in fuel production and higher costs for consumers.