The most important factor is to show that your parents will return back to Nepal after visiting you/ after their authorized stay in the US. You can provide documentations that will show to the consular your parents have significant ties to Nepal socially, financially, culturally etc., that boosts chances for them getting a visa. Such documents may include any proprietary/business documents (if applicable), bank statements, office letters stating they're on a vacation and will assume regular job duties upon return (if applicable), letters from any social groups/organizations stating they're active members etc. - such documentations that will acts as proofs that they will return to Nepal.

Good Luck.