As the Wehrmacht had been fighting the Russians for the best part of 3 years and bearing in mind the Soviet reatment in places like Köenigsberg and Nemmersdorf (whatever the truth was or is) was enough to convince them to return to fight. The only problem was that Hitler had always thought that the Allies would see the error of their ways and fight the red beast but the allies had fought with them against national socialism so there would be an impression of a possible stitch up, or a fear of being used as canon practice, amongst the Wehrmacht.

Sorry perhaps that wasn't too clear but I feel that they would not trust the Allies as, in their eyes, they had been responsible from preventing the Nazis from crushing the Soviets and then to accept what the great dictator had been saying from the beginning that the Soviets were the true enemy won't just have caused problems at the time but would have sown the seeds for a greater problem in the German lands later. In order to allow the Germans to participate would have meant re armanent, and realignment and would have allowed the Nazis ,who only hid during the last days of the war, to reappear as renaissance figures for Western Europe. perhaps the whole point of war would have seemed a little pointless.