I think you are confusing some concepts here.

Ethnic nationalism, ie the idea that it was important for each (European) ethnicity to have its own state, was extremely common post WW1, it was championed by American President Woodrow Wilson. This followed on earlier ideas of Romantic Nationalism in Eastern Europe, leading to the founding of many national states such as Greece, Bulgaria, Poland, Czechslovakia, and Hungary where previously there had been multi-national empires such as the Russian, Ottoman, and Austro-Hungarian Empire.

Fascism, which is a little complicated to define but at its most basic a right wing, authoritarian, and anti-liberal movement, accurately describes the regimes that took power in Germany, Italy, Spain in the interwar period. While the fascist regimes in Italy and Spain certainly had some culturally chauvinist viewpoints, Nazi ideology in Germany was unique in its genocidal nature, and unique in its racial ideology.

So maybe a better question is why did Nazism come to power in Germany, which is a question that I don't have an answer to.

Hey there,

Just to let you know, your question is fine, and we're letting it stand. However, you should be aware that questions framed as 'Why didn't X do Y' relatively often don't get an answer that meets our standards (in our experience as moderators). There are a few reasons for this. Firstly, it often can be difficult to prove the counterfactual: historians know much more about what happened than what might have happened. Secondly, 'why didn't X do Y' questions are sometimes phrased in an ahistorical way. It's worth remembering that people in the past couldn't see into the future, and they generally didn't have all the information we now have about their situations; things that look obvious now didn't necessarily look that way at the time.

If you end up not getting a response after a day or two, consider asking a new question focusing instead on why what happened did happen (rather than why what didn't happen didn't happen) - this kind of question is more likely to get a response in our experience. Hope this helps!