An excerpt from the link below:
Whedon expanded on the omission of Pym to io9.com, saying, "We don't have to have him. It works very simply — this is Marvel cinema, not Marvel comics. One thing [Marvel Studios President] Kevin Feige has a genius for is knowing what to hold onto and what to let go of. You can invoke the feeling you had and play with the characters you love and remain true to the needs of the film."
So, if Pym doesn't create Ultron in the sequel, then who do we blame? "We're crafting our own version of it where his origin comes more directly from the Avengers we already know about," he's quoted as saying in the Marvel interview. "It's a little bit darker than the other film because Ultron is in the house. There's a science fiction theme that wasn't there in the other one. Ultron is definitely something that evolves, so we're going to get together a couple of different iterations. Nothing can be translated exactly as it was from the comics; particularly Ultron