Marketing, pure and simple. When people see the latest color schemes it gets them to thinking that they need to trade up so to appear to be current. Can't show up with an "aged" RV. When the market becomes saturated with a color, the manufacturers will turn to a different color scheme. Just one way, along with all the bells and whistles they keep adding, to encourage us spend our money.
Full body paint schemes are another, but those are expensive enough options to keep many people away.