Here's why:

* Standard state refers to the most stable form of an element under standard conditions (298 K and 1 atm pressure).

* Sulfur exists in several allotropes (different forms with the same chemical composition). The most common and stable form at room temperature and pressure is orthorhombic sulfur.

* Orthorhombic sulfur is a yellow crystalline solid with a cyclic S8 molecule structure.

While other allotropes exist, like monoclinic sulfur or plastic sulfur, they are less stable under standard conditions and will eventually transition to orthorhombic sulfur over time.