As far as I know, there are two valid reasons for denying visa to parents wanting to come for graduation. 1) if the counselor thinks that the parents don't have strong ties to the home country and there is a chance that they might try to stay in the US 2) if the counselor feels that the student or parents aren't financially capable of sponsoring the trip. Since your brother is in Nepal, I don't see why the counselor would think that your parents would try to stay in the US. So, may be it's the financial thing. I am just speculating, though. You should definitely ask them to try again and emphasize that your brother is in Nepal. Since your mom is already retired and dad has his own company may be the counselor felt that it will be easy for them to just stay in the US and not return. Also, you could ask someone in your international student office to write a customized letter for you; I don't know if that is going to help but I can't see how it would hurt. Please post your experience after the 2nd try. Good luck. !!

Edit: I completely agree with OC101's comments below. I think that is the reason they were denied visa. It's funny how what you thought were your strengths are actually your weaknesses as far as getting your parents the visa is concerned.