SS doesn't come in SG (Space Gray). Also SS is a premium over aluminum. You can buy an aluminum and then buy any leather band far cheeper than a SS with the band. And have an extra Sports band to boot.
The sport model Apple Watch really isn't targeting fashion as much as it is for fitness when compared to the stainless model, which Apple offers more expensive band options paired. And to keep the Sport model price lower , they obviously offer it with a sports band. If you want A leather band, you just have to purchase it separately. Otherwise, leather typically pairs better with stainless.