for a few reasons... Mostly because Iron Man 2 and 3 were both pretty bad, IMO.
Additionally, RDJ's Tony Stark character has become obnoxious and annoying in more recent films. He creates Ultron, then lectures everyone in Civil War about how they need to be held in check- but doesn't have the self awareness to realize that he is the cause of most of the Avenger destruction. He also confiscates Captain America's shield and Spider Man's suit because they area doing what he wants them to. To be blunt, his character is a jerk and should be killed off.