Because it's a $6-8 item? Seems like the fast food restaurants advertise their expensive items more than the value menu items and I imagine it's because they want to get you hooked on the higher margin items.
RE: Arby's - to show you how old I am, I remember (and the last time I ate at Arbys), the regular roast beef sandwiches were 5 for $5 (perfect for dinner for a family of four with mom dad sis and me). I think I saw in a recent Arby's advertisement that those regular roast beef sandwiches are now 2 for $5 or something like that.