Because Nazis don't threaten capitalism, whereas what defeated the Nazis does.

It's worth noting that fascist organizations existed in much of Europe (and more) before WWII, and many are capable of selectively choosing the parts of Nazism they liked while ignoring the parts where Nazis called their countrymen subhuman.

When liberalism fails and the worker class turns to socialism, the privileges of the upper- and middle- bourgeoisie (the upper classes in terms of economic relations, rather than in terms of wealth) are threatened and so in reaction they either form or fund fascist movements to replace liberalism without losing capitalism. The rise of squadrismo in post-WWI Italy is a useful case study.