By making a skiller you willingly limit yourself to a certain amount of content you can experience, it would make no sense to encourage this by making a skiller specific max cape. As a skiller you already get to wear combination capes and 200m capes, you have plenty of capes to show you're a dedicated skiller. Mains would get to wear this cape too, which wouldn't make sense for a cape designed for skillers.
If you want a max cape just train combat, it won't do you any harm. 126 always looks better than 3, and you won't get killed by things when you're just trying to skill. A second maxed main is always more useful than a skiller.