After both Varus and Germanicus essentially failed to make any significant inroads to conquering the area, it was eventually decided to be not worth the effort. The land offered little resources, and was a hard and cold. Tacitus deemed it as such:

Then, besides the danger of a boisterous and unknown sea, who would relinquish Asia, Africa, or Italy, for Germany, a land rude in its surface, rigorous in its climate, cheerless to every beholder and cultivator, except a native? [Full text here.]

This, in conjunction to the fierce natives, meant that not only would it be difficult to take, but to maintain rule as well.

The other primary motivation for conquest besides material benefit, eliminating a threat, didn't apply. The only time the Germanic tribes really posed any true threat to Rome was when they were united against a common enemy (such as Rome itself). There was no true established power or state, and thus no real risk of a coherent and organized invasion or some such.