@Alon Your question is really why did the Greens succeed in cutting into the Social Democrats' voting base.
That's part of the bigger question of SDs' loss of working class votes and shift to a base of the educated middle class. The Greens cut into the new base, not the old base, and in many ways their platform fits their constituency better than the SDs'.
You could even argue that the rise of the Greens has benefited SDs by pushing them back toward their labor base (see Australia).