It seems to me the Romans managed to push back celtic culture and peoples really to far away regions in part of Europe such as Scotland, wales and Ireland. I've heard accounts from Julius Caesar that celts told many Romans stories of how terrifying the Germanic warriors were? Obviously with the fall of rome being linked highly to invading Germanic tribes how come the Celtic tribes failed where the Germanic ones prevailed ? tl;dr: how were the Germans more successful than the celts at fighting rome and eventually conquering it?
There is actually something of climatological boarder between France and Germany due in part to France's exposure on the west to the Atlantic ocean and the warming effects of the the gulf stream. This would have put Gauls climate much more inline with the western Mediterranean Norms. See Map
This difference would effect the rate of spread of agriculture, and therefore population density and social orginization. Then there are the Alps, which also create a formidable natural barrier between Northern Italy and Germany.
The combined effect, was that it wasn't until the reign of August, that the Empire was really able to start projecting power into Germany (see Quintus, and Germanicus), but on the other side of the coin nor was Germany that tempting of a target, with few cities and limited wealth.
By the time the situation stated to change in the 3rd century, the Empire was no longer expanding.
