Hyphae are harder to grow in the unpeeled soybeans.
If you make tempeh with unpeeled soybeans, the fermentation process won't go well. With the skin, the finished tempeh can be too fragile. Even if you touch it with little force, the hyphae on the beans and skin are easy to tear apart. Therefore, it is not recommended to leave the beans unpeeled if you want to make good tempeh.