This phrase has nothing to do with the former Iranian monarch: you find it because “Shah” is a common name and people who bear it are a large target market.
Many businesses engage in selling ‘personalized’ merchandise—a t-shirt or hoodie or coffee mug or some other sellable object imprinted with —the name of the buyer or of a group with which the buyer identifies—or with a catchphrase into which the name can be inserted.
It's a X thing, you wouldn't understand is one catchphrase popular with these merchandisers—presumably because it’s popular with their customers. The phrase means “That is a matter which only people who are X understand; since you are not X you cannot understand it”. It thus asserts the superiority of people who are X to people who are not X.
Such merchandise is usually available for just about any name, and marketed with internet links and images which embody most fairly common names—“Shah”, for instance. I was unable to find any merchandise for “kantx”, but here’s a website marketing hoodies and t-shirts with the name “Smith” inserted into many different catchphrases.