Two reasons. Leafy greens are easy to grow hydroponically because they grow quickly. The problem with them is that they have nearly zero calories.
People invest money into growing them hydroponically because they don't keep, or transport well, which makes them incredibly expensive out-of-season. The high margins make it economical. Nobody's going to be growing rice hydroponically, for instance, because rice retails for 75 cents a pound.
Size is the main thing, plants like corn haven't had much in the way of vertical constraints so until we adapt new shorter breeds of them you can only grow so much per cubic meter and the economics doesn't work out compared to an open field. There's also the economics of how much goes into the non edible parts of the plant, this is much lower in leafy greens. Aside from that they just don't keep as well as more robust crops, so they have to be a lot fresher.
