I'm not sure Theresa May gets any real credit for Brexit as a concept. She didn't campaign for it; she was just daft enough to think anyone could negotiate something that satisfied the multitude of unworkable or contrasting promises.
Another take: She didn't realize that Boris Johnson wanted power, and therefore would do everything possible to undermine her. There was no way to make a deal with Johnson.
An electoral system where the majority of votes cast were not in his favour