After WWII Germany wasn't really in a position to deny anything. Like it got a defeat without conditions, the country was occupied entirely and split between 4 countries, many concentration camps were liberated by the allies themselves so they were still intact and told their own story. So unlike many other countries the deeds and their paper trail was so well conserved and available to the global audience that there was really no point in denying that.
Not to mention that both newly found German states were front lines of the cold war and were integrated into their respective blocks which had both been enemies in the war who were themselves on the receiving end of that or at least had full access to those facts, so there was no reason they should take any bullshit on that.
The allies also attempted a de-nazification, however the success of that seems to be vastly overstated. Like in the USSR occupied zone there was an emblematic narrative of "we are the antifascists", while fascism was the direct consequence of capitalism and so anti-fascism was really more of an anti-capitalism and the imagery serving as de-nazification disregarding individual involvement and consequences of that. So perpetrators became part of the resistance and fascism is a universal problem no relation to the individual.
While the western allies had a more ambitious programs (each one their own with varying grades of ambition), especially the Americans arguing with a 50 year plan to re-educate or "at least a generation of occupation", however by 1948 the U.S. had already lacked interest in pursuing the combat against latent nazism, especially the minute investigation between "follower" and "perpetrator" and instead was more occupied with the Eastern Bloc.
And after that Nazis basically rolled back all of that. Like seriously they banned the death penalty (not bad per se but dubious in context), pardoned those judged by the allies, reintroduced officials and higher ups into their former positions, passed laws that included assistance to genocide on the list of crimes with a period of prescription (so that their crimes just based the period without any legal consequences) and didn't pursue or actively discouraged processes against Nazis. Reparation largely went to German victim and those western allies which couldn't be avoided, no reparation to Eastern Europe despite the most damage being done there. While generally applying a "forward facing attitude" and an unofficial "don't ask don't tell" policy in terms of personal involvement in the Nazi regime. So in effect former Nazis were reintroduced into society as if nothing ever happened.
While victims of the Nazi jurisdictions had to wait until 1998-2009 for nazi era convictions to be pardoned. Like convictions for defecting soldiers were legal until 2009 while de-nazification judgements got pardoned in the 50s and 60s.
Like seriously the list of former Nazi party members that were politically active after 1945 is long:
While the processes against Nazi perpetrators came late took long and led to few convictions. With few exceptions it took until the next generation and the late 50 and a new resurgence of anti-semitism to be contrasted with first attempts to investigate and educate about the Nazi era. By the late 60s to early early 70s there was a clash between the generations and the process of dealing with the Nazi era was an important part of that and by the 80s and 90s there were already many who wanted to put an end to the remembrance of the Nazi era crimes. So anything but "de-nazification" being an overwhelming success story.
The bitter irony is that the alienation of the de-nazification process and the problem of a clear demarcation line between a perpetrator and follower, lead to serious war criminals getting grouped in the same category as actual followers creating a larger group of people with a sympathy towards amnesty as the feared personal consequences for themselves because of their history. Not to mention probably a demand of competent personnel which was hard to find without a background in Nazism. Like the former Wehrmacht and secret service who were heavily involved in the execution of Nazi crimes, mostly got a pass due to being necessary in terms of anti-communism, which also added to the picture of serious criminals getting a pass while lower level contributors being made an example.
So reintegrating and rehabilitating Nazis defused that problem, but at the price of them letting them get away scots free with a genocide and giving them power and political influence, again. Ironically it was probably also easier to accept a "collective guilt", if that meant that there is no individual guilt and no personal consequences. Though the attitude of the war generation apparently actually was to just keep going and not talk about it. So it's really more the success of the following generations that meant a break with Nazi era traditions and pushed for investigation and education.
PS: Another point why the de-nazification was probably more likely to succeed was prosperity. Like being able to rebuild and gain prosperity outside of theft and in combination with a different system probably helped to erode the conservative narrative of "in the good old days everything was better".
So tl;dr maybe a combination of the inability to deny and create a revisionist history on the spot, due to overwhelming evidence and reliance on the former enemies as partners in the proceeding decades, in combination with a new generation that demanded answer to the questions rather than proceeding with the silence?