To 美國老土 or any experienced friends. Thanks

I recently got job opportunity and the potiential employer have interviewed me. The interview went nery well. They are considering me. The employer provider full trainingn although this job is nothing to do with my major. So after the interview, the employer email a question " I did forget one very important question while you were here at my office.  I never did ask why you are not employed in the your  field for a company, university or nonprofit sector?  I would assume a position in your field of study would be most lucrative." This job is really not in my field but I worked in some universities for 8 years in US. How to answer this question? Thanks a lot.