The more you think about it, the clearer it will get. Here are some pointers:
1) Leftism is about collectivism, and collectivism is about sacrifice of the individual to the needs of society, exploiting the strong for the benefit of the weak, the many dictating over the few. Conversely, European culture has evolved in the opposite direction - recognizing the individual as the center and thus deriving individual rights and personal freedom. The two world views are inherently incompatible. The Nazis, while ultimately socialist, partly promulgated Nietzsche and the strive to perfection as one of their slogans. Thus the modern leftists think the Nazis were individualist, and since Nazis are bad, so is individualism. If the Nazis did not become radioactive, individualism would not have contracted it either.
2) Many upstanding anti-communists joined the nazis to fight communism and thus died. If they had not, they would be around to offer a great counterweight to leftism taking Europe and America by storm.
3) WW2 and its devastation formed a horrific image in the minds of the sheeple that ANY war is inherently unjust, evil, and unacceptable. They drew the incorrect conclusions and so did culture in general, and the vanquished hawks were not around the counterweigh. We see the failures of this attitude both in Vietnam and Iraq. The same is true for "casualty-tolerance".
4) A great many conservatives and anti-communists, who did not join the nazis, still ended up jailed and killed off by the commies in a "internal security" wave instead of war action. Nobody counts those it seems. They would have been a counterweight in each of their countries.
5) In terms of self-loathing, every time anybody is to express a preference for Europe or western culture above all, he is 50% likely to have "racist" and "nazi" thrown in his face. That alone is damaging enough. Add to that teens that feel compelled to behave like minorities "to blend in" and out of "white guilt", and you will see how the nazi defeat and the rise of multiculturalism have made being a "cracker" essentially radioactive. How can anyone promote/maintain/preserve a culture when there is an immediate negative feedback? Why isn't the same feedback given to the "underprivileged" cultures? All of this is psychological and political fallout of WW2.