The wheel manufacturers that we buy from say it has to do with showcasing the actual wheel design. With a super deep dish and large surface area to attach color i.e. Powder coat, it draws a lot of attention to the design of the wheel rather than how big the tires are.
I personally can't stand the style. It has brought low rider influence into the full size truck world and just defeats the use of a truck to me. Though who am I to argue with a business making killer money on products?