How did Cawthorn beat a primary opponent who was endorsed by the establishment, Trump, and many Freedom Caucus members?
From my understanding as an outsider who reads blogs:
1) Meadows didn't file on the last day of filing, so that his realtor friend could have a relatively clear path to the nomination. Many people in the district saw this as unfair and held it against Bennett.
2) Bennett just had really, really bad campaign skills. It didn't help that she went against a charismatic, handsome kid like Cawthorn, but her skills were just objectively bad.