Your reply should go beyond the basic because-that’s-what-interested-me-most answer.
This question is less of a potential minefield than some other interview questions only PhDs get asked, since employers generally only ask out of personal interest or if the subject of the dissertation is relevant to the nonacademic job at hand. Still, don't brush off this question. Instead, explain how you felt that previous research on your topic was insufficient and that you embarked on your project to help scholars better understand it. Doing so will paint you as a problem solver—someone who can identify and fill a need that others might be unaware of—which is one of the most valuable traits that employers seek.
If applicable, also describe how your research methodologies will potentially help other scholars in your subfield or academic discipline. In so doing, you’ll show potential employers that you have the ability to add value by pushing the collective needle beyond the status quo—an even more desirable trait in any job market.
Example Answer #1: I conducted research on Native Americans in the 18th-century Southwest not only out of personal interest—I grew up in Santa Fe—but also because it’s still one of the most under-studied regions and periods in American history. There’s still so much we don’t know. I chose to study Navajo and European understandings of disease in particular because I believed that by doing so, we could better understand how ideas related to health shaped experiences of colonialism in the region. While my research explored perceptions of health in one place and period, the way I was able to reconstruct Navajo ideas on disease and medicine has the potential to help historians do the same for colonized peoples in other places and periods who left few written records.
If your dissertation is in any way related to the alt-ac job you’re considering, make that connection explicit in your answer as well. If, for example, the applicant in the example above were applying for a position in a health-related field, she could conclude her explanation by saying, My research also got me interested in the ways in which people today time think about health and medicine, which is what led me to apply for this position.