When I read about history, it seems clear that hills are not very good for farming compared to flat land. Especially in Europe, hills and mountains seem to be reserved for tribes or poor populations who were not wealthy or powerful enough to occupy better farmable land for themselves.
I tried searching on Google about what exactly makes uneven land so bad for farming, but all I found were articles about modern hill farming and terrace farms, none of which really answers what I want to know.
So in short, what physical characteristics of hills makes them bad for farming crops? Does this apply to all crops, or just the most profitable types? Does it have to do with uneven sunlight, bad soil, rocks, water flow...?
Water flows downhill, first of all, so drainage is a real issue. And yeah, you're going to get uneven sunlight. But most importantly, you can't plow at 45-degree angle, so large-scale hill farming is almost impossible. Preindustrial agriculture (at least in Europe) was conducted in partnership with draft animals - horses, donkeys, oxen - that were harnessed to a plow and used to cut deep furrows into the land, into which seeds were then sown. This process won't work at an angle, so you've got to do it by hand. That's difficult, time consuming, and doesn't work nearly as well (remember, you're still at an angle), so you're going to get much lower crop yields. That's why people practice terrace farming - cutting flat portions into the hill to enable even seeding and straight furrows. It's still harder than farming flat land, but much easier than farming at an angle.