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 m

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.