Since "Ant Man" is getting his own movie directed by Edgar Wright and it will be a different franchise they decided to drop the Hank Pym character and handed the creation of Ultron to Iron Man (since "Jarvis" is an artificial intelligence already i think it will be "easier" to the audience to "accept" the fact that Tony is building a "better" and "improved" version of Jarvis as Ultron)
I hate the idea and found out it was very forced but what else are you going to do? we're talking about the guys that decided that The Mandarin should be joke...