Material Edge of the USA after the war.
A huge amount of the perception that the Axis had a shot comes from the purchasing power of the American consumer, a consumer who for a list of cultural reasons, likes tales of even fights and underdogs. They dislike the idea that the US were basically Russia plus brains and minus NKVD. The Americans can afford to win through throwing bodies on a fire like the Russians; they have the society and organizational capacity to do this intelligently in a way that does it with far less brutally than the USSR. Combined together, this makes them unstoppable. But we don't like looking in our mirrors and seeing the Borg, even if its what allowed us to triumph.
So you get cable channels that fluff up the Nazis and make the war a more even fight, books and games that really pile it on, and a consuming audience eager to accept these exaggerations and tales. Because we must be the underdog, or fighters in a fair fight. And the post-war boom means that the men who fought the war are a consuming audience for decades - if they want to hear about their youth, they want the heroic tale. Who wouldn't?
The hagiography makes an interesting contrast to the accounts of the Allies' commanders, administrators and planners. They knew they were smothering the Germans with men and material, and made no bones about it. These boards have more people who drill past the hagiography and into the facts. Plus, there's a considerable personal pride here of being the people who study the "real" story, and don't just go for the common myths and simple stories.
In addition, there's a distaste for the rule-of-cool factor that runs so much of the interest in the Third Reich's equipment and military. So the Wehrmacht had cool kit that anticipated a lot of modern military gear. So the Wehrmacht's uniforms were cool. So what? They were still serving on of the more vile regimes in history. Add to this some of the deeper attitudes of some reenactors, which are just not fucking seemly. They have other places on the internet to fester. Not here.
So it's part of the culture of the boards, and the verdict of an overwhelming amount of the historical evidence.