in addition to the very good sibling comment: lettuce is also a cold-tolerant crop that can grow well at 50F, whereas other popular consumer produce like tomato

in addition to the very good sibling comment: lettuce is also a cold-tolerant crop that can grow well at 50F, whereas other popular consumer produce like tomatoes or peppers require minimum soil temperatures of 75-85F for optimal growth. Managing that optimal growing environment gets costly quick. Another important factor: lettuce grows uniformly, and the whole plant is harvested at once. There is no pollination required, no trellis-training for production vines, or fruiting bodies that must be plucked when ripe. Lettuce is about as straightforward as you can get in terms of mass production.