Why does Art Spiegelman use mice instead of people to portray the characters in the story?

Spiegelman's use of mice to portray the Jews is a play off the Nazis considering them vermin. Spiegelman isn’t making fun of anyone here, he is attempting to use ethnic stereotypes to break down the allegory that some people feel or felt was true. Does that make sense? He was using these stereotypes as a platform to show people the truth.

It was of course not just Jews and mice. Different animal characters play various other cultural stereotypes. The Germans are cats, predators who prey on the Jewish mice; the Americans are dogs who save the Jewish mice from the German cats. The French are frogs, and the Gypsies are moths. The Poles are pigs; the Nazis actually called them Pigs back in the day (I have no idea why).

I’m not a historian but I think the use of animals also gives Spiegelman an accessible way for readers to handle the horrors that took place. Those are my own words so you might want to paraphrase this.