Also, audiences love James Bond, a suave, quick-witted playboy type who relies on his wits and the occasional gadgets. Tony Stark is right in that mould. He will be more fun than Captain America, who has the weight of responsibility on his shoulders or Thor, who could seem too otherworldly, while Hulk is just unrelatable to the general public. Iron Man gets the girls, the fast cars, the gadgets, the parties, the money, the fame etc. Now tell me who you would want to be out of that group of heroes?
Exactly, this standard stock character of the suave playboy who doesn't play by the rules, but gets what he wants is just... popular. Not saying he shouldn't be, but it's just the truth.
Cap will be a boy scout, Thor will probably disagree with the affairs of man and have to "come around" and Hulk, no one wants to admit they have inner demons, no one wants to be the guy on the run or lash out and hurt people.
GA generally for action movies such as super hero flicks, wants to be the guy who gets **** done with little to no effort. In Iron Man, the terrorists don't break his leg, fingers, or anything. They dunk his head in a tank over and over. While still technically torture, and is probably excruciating, he seems to come out unscathed. Builds a super weapon right in front of their cameras putting braces on his leg and everything and somehow is infinitely smarter than terrorists who go "Oh yeah, don't worry that is definitely a missile." Flys into a military zone, blows up two jets no repercussions. Fights a huge stronger robot, beats it in a matter of minutes. Gets to bang the reporter who despises him the same night as she insults him to his face, and goes out gambling with super models the night of his award ceremony. Even when he finds out his is responsible for the persecution of innocents, he goes over there and cleans up that mess with little to no problem what-so-ever, almost taking out an entire guerrilla force. In the sequel he's going to take down robot after robot (probably also showing up War Machine) with quick quips, defeat his rival, his villain, and give the finger to congress and come out unscathed. All with no powers and a suit. This is the guy people are going to want to be.
Hulk on the other hand has trouble with one man (Ross,) and has hissy fits that kill people that Banner has to live with.
Cap is going to be frozen and wake up 60 years later with people he knows and loves aging and dying, has to play by the rules, and is a general stick in the mud to Tony's partier attitude.
Thor will probably be disinterested and looking down his nose on mortals at first.
These really aren't the people that the GA is going to generally care for when put on the same screen with the "ideal man." Now, does that mean it won't create some awesome team conflict? You bet your butts it will (hopefully,) but it also leaves the GA siding with Tony who just has less trouble than everyone else.
It's like RDJ's other franchise right now. You don't see anyone really going "Aw man, Watson is the ****." No, they want to side with RDJ's character. Brilliant, funny, handsome, charming, gets crap done with unbelievable odds.