The main issue with your parents petition is with you: By being only one son.
When the immigration officer learned that you are only the son for your parents its trivial that your parents are going to stay with you in US(thats what the immigration officer interpretation, I am sure its not valid from your words).
Your parents can apply again and this time by being careful in giving details about you(if asked only tell the truth)
Your parents must have more evidences proving them to come back to India(proofs like ongoing business, fixed properties, other ties back to the home country). Its a gray area, and it depends up on many variable factors.