I remember watching a short interview with Simon Wiesenthal. He was telling how in the 60s, during a conference in Italy about the Holocaust, an old widow asked him if he only searched for those who killed jews or also other ethnicities. Wiesenthal answered "I search every criminal". So, the widow recounted how her husband was killed in a massacre that Wiesenthal never heard of, the Cephalonia Massacre.
The massacre was the nearly obliteration of the Italian 33rd Infantry Division Acqui. After the 8 september Armistice, a small firefight happened between German ships and an Italian artillery battery. What followed was a battle that lasted seven days. After the Italian soldiers surrendered after running out of munitions, the now prisoners were executed and the survivors loaded on ships to be deported in Germany. Those ships were then sunk by the allies. In numbers, 1315 Italian soldiers died in battle, 5155 were executed and about 3000 drowned; around 200 survivors spent the rest of the war doing forced labors.
When Wiesenthal tried to open an investigation about the massacre at the Dortmund's attorney , he clashed with a wall. Then he undestood that it was not the SS who committed the crime, but the Wehrmacht. And "the Wehrmacht was a sacred thing".
The Wehrmacht committed other massacres in the Aegean territories: Kos (102 officers and a veterinarian) and Rhodes (90 execution + 5800 prisoners drowned).
I think the Germans truly needed to feel separated from their recent past. Giving all responsibility to the SS was the quickest way to exorcise the collective fault of all the crimes committed during the war. The SS, with all their hateful actions and thinking, were just the perfect scapegoat. Giving the Wehrmacht their share of guilt probably felt to tiring, socially and spiritually speaking. Once people felt confortable in this construct, the "clean Wehrmacht" truly became a myth, one that nobody wanted to dissolve.
Only after many decades, when the war started to become "abstract" (when fewer and fewer people that lived it remained) the myth was seriously challenged in Germany.