You can see, EVERYONE is here. It's a jam-packed poster. Despite concept art for Vision already released, he's still being kept to the shadows. It may not be the best movie poster but it does the job in getting you excited for the movie. There simply is a lot going on and a lot to expect. Hey, what if Ant-Man was also on the poster? Doubtful but you never know.
Here's the international version. You'll notice it's less busy and has Iron Man in the front.