Comic fans know that in the comic books Hank Pym is the man who creates Ultron. It’s because of that reason so many people were surprised to learn that Pym would not be showing up in Avengers: Age of Ultron despite the fact that he will soon be a part of the MCU thanks to Ant-Man. Joss Whedon shed a little light on the subject in an unpublished portion of an interview with Empire Magazine that took place in April 2014. He goes in depth as to why Pym won’t appear in the film and it makes a lot of sense.
“Of all the heat I’ve ever taken, not having Hank Pym was one of the bigger things,” says Whedon. “But the fact of the matter was, Edgar [Wright] had him first and by virtue of what Edgar was doing, there was no way for me to use him in this. I also thought it was a bridge too far. Ultron needs to be the brainchild of the Avengers, and in the world of the Avengers and the MCU, Tony Stark is that guy. Banner has elements of that guy – we don’t really think of him as being as irresponsible as Tony Stark, but the motherf**ker tested gamma radiation on himself, with really terrible, way-worse-than-Tony-Stark results.”
“It didn’t make sense to introduce a third scientist, a third sciencetician, to do that,” Whedon continues. “It was hard for me, because I grew up on the comics, to dump that, but at the end of the day, it’s a more interesting relationship between Tony and Ultron if Tony was once like, ‘You know what would be a really great idea?’ They’re doing what they always do – which is jump in headfirst, and then go, ‘Sorry, world!’ But you have to make it their responsibility without just making it their fault.”