>>1477082>Thermal protection on the North American orbiter concept, which would have ridden on top of a reusable first stage. (North American Rockwell)By this point, United States Air Force requirements (not that USAF contributed any money to development) required a larger orbiter to accommodate its large and heavy spy satellites. USAF requirements to be able to perform polar orbits from Vandenberg also dictated a massive delta wing to provide large crossrange capability (not that the Shuttle ever launched from Vandenberg), against the straight-wing fetish of Max Faget.
As NASA's Steven J. Garber notes,
>Given these four goals of creating a space transportation system that would 1) be reusable and thus lower the cost of accessing space, 2) be safe enough for humans to pilot, 3) have 1,100-mile cross range capability, and 4) have a significant payload capacity, NASA chose a Shuttle with delta wings that seemingly could achieve all these objectives. A straight-winged vehicle would not have sufficient cross range capability. It would be difficult to develop a lifting body vehicle or ballistic capsule with significant payload capacity.https://history.nasa.gov/sts1/pages/scota.html 