Originally Posted by Raiden
People also need to realize that the actors are contracted for a certain number of movies, and Marvel cannot allow them to appear on other people's movies and thus "used up" the movies they're contracted with. And an Avengers movie is an extremely expensive one, an event film that is more suited to show up every few years or so. If the threat isn't as big as an alien invasion like in TA, there's no reason for every Avenger to show up, either.
A terrorist attack on a malibu mansion is hardly going to get the team to asemble.. when they get there, what would they do??
To have a team waiting around, for another attack, again, not knowing where, is silly.
Stark was presumed dead, he then went and found the 'terrorist' - he could have easily contacted shield in that time to say it's under control.
If a hostage situation, with nuclear threat or a global disaster is on the cards, then you would call upon a roided up 80 year old, giant green stress attack and nordic/alien god.