The SPD failed to get a net plus in votes (even as they got a plus in their percentage of the total vote). The Greens lost across the board. The main problem for the Greens and the SPD, however, is not the Left Party, but rather failing to get enough people to show up.

Lack of participation also hit the CDU big, and cost the FDP their entry into the parliament.

The support for the Left Party, meanwhile, mainly comes from protest voters (who voted 'other' in the past elections) and from people who used to stay at home. When you couple that to a few CDU voters they picked off, their support is overwhelmingly (2/3rds) drawn from outside the left. This means that they strengthen the left much more than merely shifting its centre.

The question remains why many SPD and even more Green voters stayed at home. I don't know to what extent this is due to fear of an evil commie red-green-red alliance.