From an older answer where /u/Georgy_K_Zhukov also weighed in on the subject matter:

The clean Wehrmacht myth was something that was advanced by both the (West)German and the Western Allied governments after the war within the context of the budding Cold War. There was it's own Eastern German version of it but since the Western narrative is far more influential, I'll focus on that.

In the immediate aftermath of the war, Allied denazification of Germany was a thing taken rather seriously by the Allied governments. While every Western Allied government acted slightly different in their occupation zone and the Soviets focused on those who had committed crimes against the Soviet, almost 200.000 Germans had been arrested by the Allies in the immediate aftermath of the war and rather harsh laws instituted. While only a certain percentage (about 85.000) of those arrested were sentenced, when it became clear that there was to be a German state again in the running up to 1949, what was to become the German government under Konrad Adenauer took up the case of those arrested and sentenced for Nazi activity. They mounted a campaign against "winner's justice" and "collective guilt" to reject them and portray the whole thing as unfair.

The Wehrmacht as an organization in which millions of Germans had fought for the aims of the Nazi state was a perfect case for them. And the man they turned to in order to turn wash the Wehrmacht clean was Franz Halder, former head of the General Staff of the Army. Halder who had been massively involved in formulating the Commissar's Order for the Soviet Union (the document on whose basis the Einsatzgruppen and Wehrmacht shot millions of Jews and people suspected of Bolshevism), was arrested in connection with the July 1944 plot and had been imprisoned in the Flossenbürg Concentration Camp when the Allies liberated him.

During his time in Allied captivity, he had declared to be willing to help the Allies and was made part of the war historical study group of the US army's Operational History (German) Section. This section was charged with producing an account of the Wehrmacht's involvement in the Nazi state and its crimes. Halder and his colleagues had access to a lot of the captured German material and they set to work to produce an account that portrayed the Wehrmacht leadership as a tool abused by the Nazi state but far from implicit in its crimes. Several historians posit that it was Halder's aim to in essence portray the majority of the Wehrmacht leadership as the unfortunate victims of Hitler. Halder's and his colleagues' plan insofar played into Allied interests since in connection to Nuremberg, there was great reluctance on the parts of some involved to declare the German army or the leadership of the German army a criminal organization per se. While there were cases against the High Command, especially in connection with the hostage policy, the Wehrmacht itself was not declared criminal per se.

The newly created Adenauer government seized on this fact in 1949 and used to their great advantage with the German population. As I wrote above, millions of Germans had been part of the Wehrmacht and nobody likes to see themselves as criminal so this idea of the Wehrmacht as an apolitical tool full of people who had just done their duty and nothing wrong hit all the right chords.

The whole narrative really got the propagandist push it needed when early on it the German Federal Republic's existence, the question of an army was raised. With the war in Korea escalating, the Western Allies sought to rearm Germany in order to have another ally in Europe against the thread of Soviet invasion. In 1950 the Adenauer government and the Western Allies began secret talks about German rearmament.

For these talks, Adenauer conveyed a panel of former Wehrmacht officers, many of them implicated in the July 20 plot but also including notable war criminal Adolf Heusinger, in order to formulate a policy of German rearmament. The document they produced was the Himmeroder memorandum, in which the conditions for and the form of the future German army was laid out from their perspective.

The first part of the Himmeroder Memorandum includes the political, psychological and military conditions for German rearmament. The Adenauer government claimed it essential that the Western Allies stop the "defamation" of the Wehrmacht, Waffen-SS and its members, that all members of the armed forces sentenced for war crimes be released, and that all acknowledge that they acted in accordance with orders and the German law of the time.

While it took some time until the Allies agreed to these provisions (and they didn't agree to all of them), this was to massively shape German policy vis a vis the Wehrmacht for years to come. Soon after this, the Adenauer government founded several study groups consisting of former Wehrmacht members that were to influence how the history of the Wehrmacht was written in German academia and whose job it was to get former important Wehrmacht members to write revisionist autobiographies pushing the clean Wehrmacht narrative. And while the Allies acquiesced somewhat when it came to stop "defaming" the Wehrmacht, in Germany, this policy was hugely successful.

Pretty much every book of a former Wehrmacht general was written as part of this effort. All the shitty autobiographies people for some reason still read, from Panzerleader to Aus einem Soldatenleben etc. All these were part of a concentrated effort by the German government to exonerate the Wehrmacht and its leaders.

So, when asking about the clean Wehrmacht myth, there is a certain from below social dynamic to it when it comes to millions of former members but one also needs to remember that part of it came from a concentrated German government effort to paint the Wehrmacht in a positive light that did within the context of rearmament receive help from the former Western Allies.

It is also important to note that all these books still in circulation, whether by Guderian, Manstein or others, were written by literal shills for clean Wehrmacht.

It took until the 80s and 90s to fight and root out this narrative from German society and some parts of it like Rommel are still around despite the fact that the historical circumstances have changed so greatly. But all this makes me plant my feet firmly in the camp that people still pushing clean Wehrmacht either have a rather nefarious political agenda or have not delved deeply enough into the historical material to make any kind of informed claim on the matter.

Sources:

• Klaus Naumann: Die „saubere“ Wehrmacht. Gesellschaftsgeschichte einer Legende. In: Mittelweg 36 7, 1998, Heft 4, S. 8–18.

• Omer Bartov: Hitler's Army.

• Detlev Bald, Johannes Klotz, Wolfram Wette: Mythos Wehrmacht. Nachkriegsdebatten und Traditionspflege.

• Wette, Wolfram (2007). The Wehrmacht: History, Myth, Reality.

• Shepherd, Ben (June 2009). "The Clean Wehrmacht, the War of Extermination, and Beyond". War in History. 52 (2): 455–473.

The reasons are manifold, but the prosecution of war criminals in general and the Nuremberg trials in particular play a major role. In the main trial of Nuremberg, besides some of the main Nazi leaders (such as Göring, Heß and others) certain institutions were also on trial: SD, SS, Gestapo, the Hitler cabinet and the Wehrmacht. The Wehrmacht, however, was not determined by the judges to be a "criminal organisation". Unlike the SS for instance, that meant that solely being a member of that organisation was considered a crime. The original idea was to have a separate trial concerning the Wehrmacht and the crimes committed by members of the Wehrmacht. Such trial never took place for lack of funding and staff, as the allied prosecution lost steam after the main trial at Nuremberg.

In fact, after the Nuremberg trial, a separate trial was undertaken against the Einsatzgruppen, and some of the officers responsible were sentenced. But here comes the real problem with all post-war trials: People and institutions were tried on behalf of tens of thousands of others, who had commited the crimes. Often, officers, department leaders or political figures were arrested and tried, but many of the men who had their finger on the trigger just returned home as "ordinary men". When the big names were convicted, many aussmed that stood in for everyone else, thus clearing then from any crimes.

And there comes the third problem: Many of the big names in Nuremberg and later trials based their defence on a notion of duty, carrying out orders and generally being just a little wheel in a big machine. Even Göring attempted to convince the judges that he had no idea of what was happening in the east, many others followed suit. Such language carried on and eventually people just believed it because it was repeated long enough.

Lastly, as Germany needed to rebuild and be rebuilt and as the western allies needed a reliable ally against communism many issues were simply brushed under the carpet.

For those who read German, I highly recommend a brief history of the "Nürnberger Prozesse" by Anette Weinke.

MNT is the de-facto standard collection on the Nuremberg trials, but many others exist, including many on the reception of the trials in the German and international public. I am only familiar with German titles, will post if requested.