And for a good reason. What you have to keep in mind is, the small parties (Greens and FDP) have to constantly fight for survival, since there is a 5% threshold on federal and most statewide elections - fall short of that threshold even by one vote, and you ain't gonna be represented in parliament at all. And FDP and Greens are direct competitors. So entering a coalition together is a highly dangerous move for both of them. It's almost a death match - if one of them were to benefit from such an alliance, the other would likely go under.

If you can't convince them, confuse them. (Harry S. Truman)