Why are some supplements suitable for vegetarians but not vegans?

This is because it contains Vitamin D in the cholecalciferol form. Vitamin D3, (also called cholecalciferol), is typically made from a substance derived from sheep’s wool (lanolin). This is illuminated by ultraviolet light to form Vitamin D3, mimicking the reaction that occurs in our skin. (It is considered Halal because the sheep’s wool is from a live sheep).