But it would've been really offensive to have it seem like Cap played a big part in the war on the Axis instead of the normal soldiers. Having him fight a group like HYDRA (who WERE all Nazis until they broke away) keeps the potential offensiveness down.
But it's not like Captain America is Superman. He could not possibly do it all. I think the audience would understand that. And I do get that Hydra broke away from the Nazis, it's just that they didn't look
like Nazis. And quite frankly, I thought their costumes and gear were a tad over the top. It's like I was watching a cartoonish version of WWII.