From the very Start in Star Wars although it is not in the shot somehow, when Ben get’s struck down his Robe catches alight… Then Vader Stamps out the flames with his foot. The lore is established.
Uhhh … I think you’re creating that scenario in your mind, pal. There’re no smouldering/burning robes in that shot; Vader’s stomping the robes in confusion over where Ben’s remains went.