The ignorance here is astounding.
Britian had a larger population, as in large settlements in the tens of thousands, fortifications, equal to superior metallurgy, roads, the headquarters to a religous establishment that influence behind Roman lines in Gaul, and developed industries like tin and currency in the form of iron bars as well.
Germany had slash and burn agriculture and nothing bigger than villages. They barely had enough iron to tip their spears. They won Teutonberg Forest in part because they were so primitive, with Romans being spread out on a narrow forest path rather than on a proper road where they could consolidate into proper units.
Germania's poverty (in the 1st century, this changed later) had a lot to do with why it wasn't conquered. One major defeat meant it wasn't worth the expense of resestablishing the province. The intended province was only the area from the Rhine to the Elbe, so they had less strategic depth than the Britons. Retreating meant running into tribes who were rivals, not natural allies.
A lot of people on these forums seem to lump all "Barbarians" as being the same and having the same level of technology and being a vaguely primitive "other". This is an idea that just needs to die.
Britain was hardly a profitable province, at least compared to other wealthier provinces like Gaul and Spain. It wasn’t conquered for its riches, but to enhance the emperor’s glory and expand Roman hegemony. I’m not saying Germany was richer or more advanced, and I was even careful in the previous post to distinguish “barbarians” in the continent from Britons. But, being separated from the mainland is only an advantage as long as you have a strong navy to repulse enemy attacks, and even Roman liburnae could smash trough the Britons’ navy by imperial times. If a German tribe could at least move from the Rhine to Frisia, like the Chatti, a British tribe was trapped in the island, with no option to go anywhere since it was also surrounded by hostile tribes. The only option would have been to unite into a huge confederacy and battle the Romans through guerrilla warfare, but Boudicca’s rebellion proves that such large confederacy needed to give battle as soon as possible, lest it broke on its own, and no barbarian tribe could seriously hope to win against a prepared Roman army. Boudicca got trounced by Paulinus, and she outnumbered him. It’s telling that the most successful usurpers were always proclaimed in Britain, that there never was a British senator, when almost every province had at least one, and that Rome never put much of an effort in defending Britain in the fifth century, once a barbarian tribe with a decent enough navy came in the picture. As a province, it just wasn’t worth the effort.
Germany wasn’t worth conquering in the first century, but doing so in the second and third became almost a necessity due to pressure in the borders. Marcus Aurelius attempted to conquer “Sarmatia”, Caracalla campaigned extensively in Germany in 213-214, going as far as Frisia, and Maximinus Thrax also led campaigns there after Alexander Severus’ dismal performance on that sector. Of course then the crisis happened, and Rome was too embroiled in its own issues to think about conquering anything, and it’s doubtful that, even if they had conquered further territory beyond the Rhine or the Danube, they could hold it. Still, the intention was there.