After the gauls were conquered the germanic tribes were a constant troublesome nuissance. On the civilization scale to me it seems the germans were greatly outmatched and ununified so why didnt the romans conquer them all?
Hello.
The short answer is: There was no need. Gaul was a relatively civilised bunch of societies. It had coinage, detailed trade, well developed agriculture, had literacy to some extent and they were somewhat familiar since southern Gaul had Greek colonies (such as Massilia (Marseille)).
Germany,on the other hand, was unknown uncharted territory. It was filled with forests, had absolutely no relation to Roman civilisation except the times they went on raiding.
Lets inspect the instances in which they were engaged to one another.
Germanic raids in Gaul: Caesar and his Gaulic allies had a hard time defending against the marauding Germans crossing the Rhine. Although Caesar had beaten them in all instances, they were still a problem and Caesar only crossed the river once to show them a good time. Caesar was familiar with what was accross the Rhine, but in deep Germania the Romans had no idea. This all happens around 50 BC
Battle of the Teutoburg Forest 9AD By this time, the Romans had the entire Gaul as Roman territory, the Rhine river was their border with Germany. Rivers form natural borders as you may know, but there were still occasional raids by Germans. A campaign was launched to subdue the Germans. Just like Gaul, the Romans had Germanic allies as well to guide them and spend the winter with. One particular German,called Arminius by the Romans,lured the Romans into a trap and destroyed the entire Roman expeditionary force. Yes the entire force. Imagine: a barbarian people not only defeats but absolutely destroys an entire Roman army in relatively uncharted territory. This was a huge shame. The Romans did avenge this by sending punitive expeditions, but they never recovered from this loss and stayed clear of Germany.
Militarily speaking, if there is a huge chunk of land you have little idea about, then you cant just partially conquer it. Had the Romans just absorbed the one half of Germany, then they would be open to raids from all sides. If you want to conquer entire Germania then you need almost all your forces because of vast territory and you would be exposed from all sides (Remember Rome is a big country) Thats why Rhine formed a natural border and Romans spent no further energy to civilise, colonise Germans, teach them proper agriculture and trade, build infrastructure (roads were almost non existent in Germany),teach them Roman culture and many other things.
My resources include Caesar's De Bello Gallico diaries, as well as academic research on the reigns of emperor Augustus and Tiberius.