On the one hand, Caesar wanted to give reasons to explain why he had not managed to subjugate the Germans aswell. On the other Hand, he was also trying to convey why it dis not make sense to conquer the Germans for economic reasons.
Answers (1)
Rome was unable to conquer the Germans because they had no central government. Rome would have had to whip every tribe individually, while having to fight all the tribes to do it. Another problem is that the people were just more enthusiastic. They teased the Romans into sending a third of all the army, and the Germans killed them all in one battle and mailed the general's head to the senate just to let them know what was what. You can get summaries of the history at youtube by searching for Terry Jones and his documentaries about Goths and barbarians.