There really is no reason, other than balance, as we need it to be grindable for sand.
We did not yet define what exactly sand in Eco is, though, the common assumption is just "sand" as known to the average person, while we were basing it on clast size and proportion of clay, water and soil instead than mineral content. We'll definitely consider fixing gneiss, when we refine sands and soils in general.
Actually, it was always a decision between gneiss and schist and we just had to pick one to have one of all three main types of rock, but without having too many of them. Gneiss is more feasibly useful for building since schist loves to cleave.