Why does it say to avoid using the fabric softener on fluffier, cotton-based fabrics such as fleece, flannel, and terrycloth?

Fabric Softeners work by coating the fibers of a fabric to make it feel softer and reduce static cling. However, this can reduce the capacity of a fabric to absorb water (terry cloth towels) or provide warmth (fleece or flannel). As with any laundry product, we recommend following the care instructions on the item's label.