This is a common interview-rejection scenario. Definitely have your parents apply again. I know of a person, who reapplied 4 times for tourist visa, yes 4 times, the same counselor interviewed him all 4 times. The first 3 times, all they would say is that he did not qualify even though he is a leading businessman and asked him not to reapply unless there is a compelling new reason besides what he had presented before. The fourth time he asked the counselor, I just want a 2 weeks visa for visiting my relatives, that's all, Here's my business card, I don't intend to stay in the US. If you reject me again, I will reapply again. I wont give up! The counselor smiled, did not seem to have an option this time-hence, approved.  He was only granted a 2 weeks single-entry visa. The port of entry officer however granted him 6 months.He came to the US, spent only 2 weeks and duly returned back. A few months later he applied for a tourist visa again and this time they granted a 5 yrs multiple entry visa.

What I want to mention here is that if the applicant is determined, he/she should try again, and again. An aggressive yet determined approach definetely helps.

In your case, like mentioned above, they have have been alarmed by the job market issue but that is not the only thing they focused on. Your dad's company should mention in its cover letter the purpose of travel, leave approval, remuneration, position and his need to stay in Nepal to demonstrate strong economic-financial ties. Your mom's retirement should not have any impact. Additionally, send them an itinerary of stay in the US showing places they will travel as tourists besides the graduation. Show mom's pension documents to show financial ability.

Good Luck.