I'm going to guess it was an adaptation that provided a reproductive advantage. You are supposed to be very careful about getting Tomato plant leaves wet when w

I'm going to guess it was an adaptation that provided a reproductive advantage. You are supposed to be very careful about getting Tomato plant leaves wet when watering them because getting them wet makes them much more susceptible to diseases. Being resistant to such things would be a huge advantage. I've noticed such water-phobic behavior on other plants in my garden, specifically kale and broccoli.