The fact that stuff like gulags, purges, and the Holodomor were side-effects of the ideology of Soviet Communism and were not reasons the Communist Party existed in the first place as it, despite the crimes it inflicted, was founded on "positive" humanistic notions, unlike Nazism, where the Holocaust, Lebensraum, and Generalplan Ost were part and parcel of the core principles of Nazism is why this is the case.
Yeah. The Soviet Union could've easily lived up to its ideals without falling apart. If Lenin had not been such a ruthless extremist, and Stalin's rise to power had been deterred, Soviet Russia could've remained an actual nation run by workers' councils.
The Nazi ideology, however, was a system that WAS built off of the idea that one must cut other guys throat to achieve greatness.
Yeah, Nazism not just required but celebrated mass murder and Darwinian competition between ethnic groups and nations. Crucially, it didn't see diplomacy as legitimate or desirable; treaties, agreements, and negotiation were just one more tool in a universal struggle for ultimate dominance and as such made poor adjuncts to brute force at best, and the history of Nazi 'diplomacy' bears that out. After Barbarossa there was literally no guarantee the Nazi regime could offer another power that it could plausibly trust; Hitler's word was garbage and the whole word knew it. The negotiated surrender of such a power was an impossibility; no coexistence was possible as long as it retained any sort of striking power.
OTL, it was snuffed out after twelve brief years; ITTL, it staggered on another 14 at massive human cost, but in neither was it an ideology that could have been reformed.
I'm paraphrasing a bit, but Gandhi once said that Nazism is imperialistic violence WITHOUT any kind of humanitarian mission.
The Nazis never pretended they were "uplifting" a savage people, or spreading Christianity. They were like "you Slavs and Jews are monstrous vermin, and I am going to take your shit."
(RE: fascism 'working' - it's less that it worked and more that it had another decade-plus to pillage resources to sustain it. Nazi Germany's structure was that of a bandit state - what it couldn't or didn't want to create, it stole, and ITTL it gorged itself on European Russia in addition to the rest of Europe. All the bodies fed into the furnace kept it going for a while, but I think CalBear shows the limitations of that approach pretty convincingly. Sooner or later you run out of things to steal.)
What I mean by "working" is that Nazism ITTL proved, in the eyes of many, to be something that could achieve its goals.
The average person living in 1943 ITTL isn't really aware of the whole structural problems with Nazism. They are only aware of the fact that this central European nation conquered the largest nation on Earth. Going into 1953, this state is know providing an excellent standard of living for the German people.
OTL, a person who witnessed the fall of Saigon in 1975 wouldn't guess that within 16 years, the Soviet Union would fade from memory. But it was only in the 1980s that the profound economic and social weaknesses of the Soviet Union became apparent to most people. Even then, no one expected Soviet Russia to vanish completely.