Page 27 of the linked document provides a concise explanation. My own personal view is that it usually makes sense to pursue both. But with the typical college campus case that involves alcohol and “he said she said,” it is pretty unlikely a case will be prosecuted in our criminal justice system absent physical injury, witnesses or video like the Vanderbilt case. Unfortunate but that is the reality.
https://www2.ed.gov/about/offices/list/ocr/docs/qa-201404-title-ix.pdf