What's so untouchable about the New Left Party that neither of the big two are willing to talk to it, i.e. to build a coalition government with the New Left Party?
The New Left Party won't be touched by Schroeder because they are his political enemies who deserted the SPD.
Merkel won't touch the New Left Party because they are mainly hard line far leftists. In the US it would be like Bush the Republican going into a coalition with the American Communist Party. It isn't going to happen.