Steinbrück lacks grounding. Without the Greens, he won't be able to hold the state and will lose it to the Union. And that would be extremely fatal - even if the SPD in North Rhine-Westphalia is pretty much a mess and ossified, a Union government would cause even more damage. I don't understand why he wants to risk that just to show the small coalition partner who's boss. Especially since the Greens in North Rhine-Westphalia are really not particularly stressful for the SPD...
At tagesschau im Internet there's the original article.