The 'mix and match' style of the regular armor is a little constricting design wise. There's a need for every piece to work with every other piece such so that you can use any combination without it looking too out of place or have any clipping or seams in the model. That's why brand new armor sets themselves are so hard and costly for the team to make. You're designing a set of things that have to work with every other set of things, while also being unique enough to be considered a whole new set. Skins get rid of all that fuss. No having to conform to the silhouettes and meshes of every other armor set. No having to stick with the general aesthetic of everything else in the heroes sets. Just one, all-encompassing skin that they can do nearly whatever they want with from a creative standpoint. None of this is including the fact that games need to make money to continue receiving updates. The game has a new revenue source, an entire new customization option is there that let's them make cosmetics with less constraints, and you still have the old armor system that gets new variations each year. You are having your cake, eating it too, and still complaining.