Because any glimpses we have seen of the movie so far featured only Hydra goons many felt that the Nazi element might have been reduced or completely removed from the story. Well according to Fred Van Lente, writer of the movie prequel comic Captain America: First Vengeance, that is most certainly not the case.
From Newsarama..
Nrama: There is the notion out there among fans that the role of Nazis might be downplayed in the movie.
Van Lente: It isn’t. Captain America: First Vengeance is chockfull of Nazis, particularly once we start getting into Johann Schmidt’s origin in chapters 3 and 4. They’re not downplayed. Schmidt runs an organization called Hydra, which starts as a part of the Third Reich power structure, and so therefore they are Nazis.
I can sort of see why people are like, they’re not seeing a bunch of guys in SS uniforms or Gestapo uniforms running around in the Super Bowl trailer— that’s because many of the Nazis in First Avenger wear Hydra uniforms.
It is not some kind of rewriting of world history. There are Nazis in the movie, I’ve read the script. It is not trying to pretend that the Nazis didn’t exist, or that World War II didn’t happen the way we all know it from history.