The real reason Marvel changed how the Avengers looked
Sadly, comic books aren't like real life. (They're so much cooler.) Lifting characters from comic books and bringing them to the big screen is a hugely complicated process, and for Marvel, that meant a redesign of some pretty iconic characters. Longtime fans will know that they did away with things like Captain America's winged hat and Scarlet Witch's … well, everything, and they did it for very good reason. In short? Realism. The impact of the Avengers is a lot greater if you think they could very easily show up on the evening news, but getting them from all their brightly-colored comic book glory to living, breathing characters wasn't entirely straightforward.
It was, however, totally worth it.