Fans on the internet have been wondering aloud: where were Iron Man and Hawkeye during the world-shaking events of Captain America: The Winter Soldier? Why didn't Steve Rogers reach out to his Avengers pals to help him in his time of need? Well, we have the answer:
They were busy elsewhere.
That's pretty much all the answer you should need, but I have some others for you. I'll break it down in terms of story first, character second and storytelling third.
Once Nick Fury is shot Steve's life goes out of control. He goes from the hospital directly to SHIELD HQ, where he is promptly attacked and must go on the run. The last words he got from Nick Fury were "Don't trust anyone," and he decides to take it seriously.
That means he can't call Tony Stark, if only because he knows that Stark Industries supplied the repulsor technology to SHIELD to make the Insight Helicarriers. Does Stark have high enough clearance to know what Insight is and to be part of the corruption within SHIELD? Steve can't be sure, and he doesn't really trust Stark as much as he'd like, even after the events of New York. As for Hawkeye - well, Steve knows that SHIELD is after him and that Clint Barton is a SHIELD agent. He can't chance contact.
Maybe you think that the events of The Avengers means Steve trusts these guys implicitly. Let's go with that - he does trust Tony and Clint. But he also knows that SHIELD, which has turned against him, is a Big Brother style organization that has access to communications intercepts and the very first thing they would do would be to monitor his friends. It is such an amateur move to go to his friends that only some random internet commenter who wouldn't last a minute against corrupt SHIELD would even consider it. Sam Wilson is a great choice for a contact because he's been so minor in Steve's life, and yet Steve feels an innate trust in his fellow soldier.
And we can easily wave away Thor, who is offworld, and the Hulk, who is a barely contained rage monster in the best of times. We can wave away Hawkeye by saying he was on a mission anyplace that wasn't the Eastern seaboard of the United States, which seems pretty likely.
The movie gives us one full flashback to Scrawny Steve Rogers, and it's to the day of his mother's funeral. He returns to his now empty apartment with best friend Bucky Barnes, and Bucky tells Steve he can stay at his place now. I'm paraphrasing here, but Steve tells Bucky, "I can handle it on my own," and Bucky reples "But I'm telling you that you don't have to."
This is a key character moment, because not much has changed when it comes to Steve Rogers. He's the same guy he was before the Super Soldier Serum, which means he's also the same kind of self-sufficient guy he is in that scene. We're meant to understand that his relationship with Bucky is the closest of his life, and this is one of the only guys to whom he would turn when vulnerable. Steve's simply not the sort to go running for help at the first sign of trouble, especially if the situation doesn't warrant it specifically. If Cap is fighting a god, he might turn to Thor. If he's fighting magic he'll go to Doctor Strange. But if he's fighting the Hydra agents who have infiltrated the organization he serves - the organization he sticks with largely because his old girlfriend helped found it! - he'll take matters into his own hands, calling for only the minimal support needed. He's just that kind of a guy.
A shared universe is a wonderful thing, but it can be overused. By keeping all of the heroes together all the time you devalue each hero individually and you also ruin the novelty of team-ups. It's important to save the big team-ups only for big threats and not for every single time a hero finds himself in hot water. The opening prologue of many 70s Avengers issues sums it up perfectly:
And there came a day, a day unlike any other, when Earth's mightiest heroes and heroines found themselves united against a common threat. On that day, the Avengers were born—to fight the foes no single super hero could withstand!
Did The Winter Soldier tell the story of 'a day unlike any other?' Not really - it felt like a big story, but one that was totally in the wheelhouse of Captain America. In terms of telling this story having Steve Rogers run to his buddies as soon as he has trouble makes him weak and useless. Having to handwave away every other Marvel Cinematic Universe character is a waste when telling an already overstuffed story - any reasonable viewer should understand the story and character reasons why the other Avengers aren't present.
This points up the place where Marvel Studios can't win when it comes to a shared universe. In comics we have come to understand that not every Spider-Man adventure is going to call the attention of the Fantastic Four, but the movie universe is newer. If the studio made a point of having someone explain why Iron Man and Thor weren't in the film people would complain that the movies are too closely tied together. Marvel has done a good job of making all of their franchises stand alone outside of The Avengers; you need to see the Avengers team up movies to follow your favorite hero, but there's no need to see Thor: The Dark World to see Captain America: The Winter Soldier. And that's how it should be - Winter Soldier shouldn't stop to deliver exposition about Tony Stark that makes you feel like you needed to see Iron Man Three.
By leaving out this exposition Marvel leaves themselves open to nitpickers who can't fill in blanks for themselves, but that's always going to happen. It's better to tick those people off than to sabotage the whole point of having separated franchises.
Back in the day they used to give out No Prizes to comic readers who came up with explanations for continuity errors or canon problems, but I don't think this is a No Prize situation. It's kind of a No Duh situation, and these are all dots we can connect for ourselves.