Also keep in mind that Hawkeye and Widow are not really too FX-heavy to present on film (comparatively speaking). With IM and Hulk being completely CG, I am sure that the idea of adding two more characters with a lot of FX budget played into the decision.
PLUS, Hawkeye and especially Widow represent the "everyman" in this story. They are highly skilled at what they do, but the other Avengers are on such a higher level than they are. See Widow's reaction to nearly being trampled. She fears Banner.
They could easily do Ultron as Jarvis going out of control or have Banner and Stark accidentally create him. I think sometimes we try to very rigidly make the movies match the source material, but as long as it is entertaining, who cares if it follows issue 367 panel-for-panel? This series has been based on The Ultimates, but that never means that they have some slavish obligation not to go in a different direction.