Why? Mainly because of the stems. Usually stems are considered Undesirable in salad. They stick out in unpleasant ways, making it hard to get bites into your mouth and then have they have the audacity to poke you. Vitamin greens stems are totally different. They are thick, crunchy and contain so much water that I would even call them juicy. I frequently graze while I'm working, and they are now my favorite snack.
Have you used vitamin green before? Did you cook it? Was it bitter? Tender?