Because the necessary genes were already in the population. Because some prey are tasted and not eaten. Because warning coloration is good for survival of the species. Because similar species were not eaten. Because warning coloration advertizes unpalatability.
Many animals have false heads or eye spots.
They help attract the opposite sex. They make the predator think the animal is a conspecific. They direct predator strikes toward less critical areas of the body. They make the animal harder to see. The attract the predators attention.