Reason 2: Film Logic
One thing that makes Iron Man such an awesome movie experience is, for the first time, the phrase spirit of the comic isnt total crap some producer uses to calm down internet fervor. The movie takes the original point of Iron Man and sticks to it. The first two movies featured Obadiah Stain and Justin Hammer as Tonys adversaries which played up the idea that a change of heart made enemies out of men he would otherwise consider colleagues. This keeps the films tightly focused on the central theme of a weapons maker turned world saver, and films tend to rely on such focus. In Iron Man 3, Aldrich Killian is introduced as the villain. Killian fits the scientific rival motif set by the two previous films pretty well. Guy Pierce plays a far more developed character than Killians comic book counterpart, who apparently had an incredible allergic reaction to, well, general existence and committed suicide rather early on in the Extremis saga. This gave the screenwriters an excellent opportunity to swing for the fences, so to speak, in terms of using the source material to create a unique film going experience. A gamble, that paid off. Whats also important to point out, is that some people have taken issue with the misuse of The Mandarin as it is not like the comic, somehow glossing over the fact that at NO POINT is the ENTIRE story like the comic its based on. This sort of blatant inconsistency invalidates the initial complaint and makes the filmmakers jobs harder in the future. The Mandarin, while very smart in the comics, is a far more physical threat to Iron Man in nature, with most their early battles literally ending in a one on one Kung Fu battle between the two (yes, comic fans wanted to see Robert Downey Jr. inexplicably know Kung Fu from out of nowhere, they were probably hoping for a montage complete with short shorts and the Eye of the Tiger Rocky III song to explain his sudden master status). Over the top, special effects laden super heroic battles are much more suited for the Avengers films, especially when you consider the format of the films when taken as a whole. The individual, character driven films allow for intricate development and depth, so that when all the characters come together they can get right to the punching and the pew pew pew without the audience wondering what their motivations are. From that perspective an energy ring wielding kung fu bad ass is a far better idea as an Avengers villain as opposed to just Iron Mans. Wouldnt a villain that powerful immediately beg the question: Why doesnt he call Thor?