I think the reason that they deny visa is because you are only a single offspring and therefore there is no strong ties or evidence that your parents will return back to the home country. You can blame other people who came here on son's graduation ceremony or tourist visa and stay here illegally applying for Asylum due to which it makes harder for legit people like yourselves to get a valid visa.

Blame is people from our country who tend to stay here illegally applying asylum even though we can live in our country well enough.

Just my two cents.

Peace

Sirus