The standard explanation for why Rome, after conquering the entire Mediterranean, plus Gaul and Britain, did not continue expanding into modern-day Germany is that the disastrous defeat in the Battle of the Teutoburg Forest led Augustus and future emperors to pull back from Rome's previous expansionist ways.

Peter Heather's book Empires and Barbarians: The Fall of Rome and the Birth of Europe, offers another explanation, rooted in geographic determinism:

Potentially highly productive, the thick clay soils of the North European Plain required heavy ploughs to maintain their fertility: ploughs capable not just of cutting furrows but of turning the soil over, so that the nutrients in weeds and crop residues could rot into the soil and be reclaimed for the next growing season. In the mid- and high Middle Ages, this problem was solved by the <i>carruca</i>, the four-wheeled iron-shot plough drawn by up to eight oxen, but at the start of the millennium most of Europe's barbarians were doing little more — literally — than scratching the surface...

The material remains thrown up by Germanic-speakers [around the year 0 CE], by contrast [with western Celts], were generally of a much less rich and developed kind. Typical finds from Germanic Europe consist of cremation burials in urns with few or no gravegoods, only hand-worked rather than wheel-made pottery, no developed metalwork style and no oppida. The general level of agricultural productivity in Germanic-dominated areas was also much less intense. It was precisely because the economy of Germanic Europe produced less of an agricultural surplus than neighboring Celtic regions, of course, that there was smaller scope for the employment of specialist smiths and artists required to produce sophisticated metalwork. And while the romans never took a broad strategic decision to absorb just Celtic Europe, the narratives of attempted conquest indicate that Roman commanders on the ground came to appreciate that the less developed economy of Germanic Europe just wasn't worth the conquest... The defeat [at Teutoburg Forest] was heavily avenged by the Romans in the years that followed, but this couldn't hide hte fact that potential taxes from a conquered Germanic Europe would pay neither for the costs of conquest nor for its subsequent garrisoning. (pp. 4-5)

I find this argument persuasive, but I'm also skeptical of geographic determinism as a historical explanation. Is it accurate that something as simple as antiquated plow technology explains the underdeveloped economy of Germania which explains the lack of Roman conquest? Or is this another Guns, Germs & Steel case of massive oversimplification?