Is it simply that we'd maxed out the Permian Basin and didn't have the technology or market incentives to explore other sources of oil?
Was part of it ironically connected to the near-total ban on US companies exporting oil from 1975 to 2015? ie since there are many forms of crude, and not all refineries can handle them equally (expensively), did the virtual full export ban disincentivize US oil production?
For reference: look at this chart of us domestic production vs oil imports over time. Screenshot attached.
Source: Sir_Vroom