I believe this statement I found on a fact check about Mary provides the answer as to why:
The book is titled, “Too Much And Never Enough,” according to The Daily Beast’s Lachlan Cartwright. Mary Trump is a psychologist and life coach who lives in New York. She has not spoken out in the media about her uncle’s presidency or presidential campaign, BUT HAS BEEN INVOLVED IN A FEUD WITH HIM DATING BACK TO 2000, when she and her brother contested Trump’s parents’ will, The Daily Beast reports.