According to Joss Whedon, "Of all the heat I've ever taken, not having Hank Pym [in the Avengers] was one of the bigger things." However, Whedon contests that it didn't make sense to bring in Ant-Man into Earth's mightiest movie heroes.
In an long interview with Empire Magazine, Avengers: Age of Ultron director Joss Whedon explained why Marvel kept Scott Lang away from the rest of the Marvel gang, and why the comic creator of Ultron (Ant-Man) would no longer be the villain's creator in the Marvel Cinematic Universe, but Tony Stark.
Of all the heat I've ever taken, not having Hank Pym was one of the bigger things. But the fact of the matter was, Edgar 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 motherfucker 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. 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.
We'll always be down with more scientists doing science in the highly fictional world of MCU. But it looks like the decision to pull Ant-Man out of the Avengers was set in motion a long time ago. Ah well.
Read the rest of Joss Whedon's thoughts over at Empire Magazine.