The original IM movie was a success primarily because of the charismatic performance of RDJ and his on screen chemistry with Paltrow. I don't really have evidence for this, but I have always assumed that Marvel just ran with the personality that he brought to the role because it was such an integral part of the character's on screen appeal.
Yep. I enjoyed it enormously when I saw it, but reflecting on it afterwards, I realized that its plot was an entirely by-the-numbers, routine superhero origin story. What made it so effective was that Favreau just used that superficial plot as a framework within which the actors were set free to improvise, and it was their charismatic performances that made it compelling.