I would assume hilly land is more labor-intensive to farm than flat land, and more exhausting to humans and animals if traditional farming techniques are used.
How do farmers plant and plow on uneven terrain? And do the furrows on a hill run uphill-downhill, or “horizontally stacked” up the hill’s incline?
Edit: I see why sloped land is more conducive to other kinds of agriculture. Any idea how steep a grade has to be before farming grain on it is more trouble than it’s worth?