Just to clarify, SKS are inexpensive, not 'cheap'. But where else can one get an unissued, semi-auto battle rifle for around $200?
I'm not sure I buy into the arguements postulated on this site by various vendors, that the supply of these rifles is limited and that low prices are coming to an end. In prudence though, I have bought several.
In response to the OP's thread question, I would say SKS purchasers are frugal (again, not 'cheap'), value-conscious buyers. Some are attracted to the historical aspect of the firearm (not I though, too pragmatic for that). And some folks just go for the inexpensive fun factor.