Have you noticed? Purple vegetables are taking over; purple kale, purple carrots, purple French beans, purple cauliflower and now purple kohlrabi. Conspiracy theorists, ideas please, before it is too late!
But of course there is a much more prosaic answer. The purple colour in the leaves, stems and “fruits” of many plants is due to the presence of a powerful anti-oxidising agent, anthocyanin which can act as a natural sun screen for the plant, something which is especially important for young seedlings and new leaves but in something like the purple cauliflower below there can be little if any advantage to the plant. Although anti-oxidants in food a widely regarded as “good” their nutritional value must be also limited in that cooking will destroy the purple colour in most vegetables (purple cauliflower is said to be an exception to this loss of colour).
Have you tried any purple vegetables? Did you manage to keep their colour when you cooked them?