Apart from the pleasing look of pure carbon twill in comparison to carbon kevlar twill of the same weight, why choose carbon instead over the stronger hybrid version? say in a car panel.

Because Kevlar isn't stronger? It totally depends on what the desired properties of the laminate are. Carbon has the highest ultimate tensile strength, compressive strength and flexural strength of any composite material. Simply put, if strength-to-weight is the goal then pure carbon is the way to achieve it.