Coulson died in the Avengers movie thanks to Loki, but Fury revived him in Agents of S.H.I.E.L.D. TV series. So, in-universe status of Coulson isn't a hurdle at all. He is alive and it has been explained in detail.
Talking about privacy, it barely matters if producers/writers want to. Nick Fury is also officially dead, but he'll be in Avengers: Age of Ultron.
Why exactly isn't Marvel Studios featuring Coulson in the MCU phase 2 movies? He isn't a part of Avengers: Age of Ultron crew, too.
There's no confirmed in-universe reason. However, Coulson and S.H.I.E.L.D. were presumably kept busy while the Avengers did their stuff in Sokovia.
Out-of-universe, Joss Whedon discussed this in a 2013 interview with the Associated Press:
AP: You brought Agent Coulson back to life for the TV series after his apparent death in "The Avengers." Is it possible he'll return for the film's sequel as well?
Whedon: He could. Right now it's not something I'm pursuing because I have so much going on in "Avengers" 2. Finding out that Coulson is alive would be an entire B story. And I already have too much movie.
Even though the comment was made two years ago, it's a pretty fair issue; Whedon has commented before on the difficulty he has balancing the narrative needs of so many principle characters. Coulson's re-introduction would add another layer of complexity to a movie that already has to introduce concepts from three sequels to the standalone series ( Iron Man 3, Thor 2, and Captain America 2), develop the Infinity Gem storyline with an eye towards Infinity War, and set up the emotional beats for Captain America 3 and Thor 3.
And tell that story about evil robots.
More recently, he made another comment in a 2015 interview stolen reported by Buzzfeed (italic emphasis is his, bold is mine):
"As far as I'm concerned, in this movie, Coulson's dead," he said. "If you come back in the sequel and say Coulson's alive, it’s like putting f***ing John Gielgud in the sequel to Arthur. It mattered that he’s gone. It's a different world now. And you have to run with that."
He's dead, Jim.
Joss Whedon, writer/director of both of the Avengers movies, executive producer of SHIELD, and adviser on everything MCU explained his reasoning recently in this interview.
A lot of people come back in The Winter Soldier. It’s a grand Marvel tradition. Bucky was supposed to die. And the Coulson thing was, I think, a little anomalous just because that really came from the television division, which is sort of considered to be its own subsection of the Marvel universe. As far as the fiction of the movies, Coulson is dead.
To Whedon, Coulson isn't alive in the movies, and thus, can't be in them.
I believe this additional information comes from SFX magazine (Buzzfeed, ever the quality news source is not clear here), but Whedon made another similar comment, further explaining why he is not in the sequel.
“As far as I’m concerned, in this movie, Coulson’s dead,” he said. “If you come back in the sequel and say Coulson’s alive, it’s like putting fucking John Gielgud in the sequel to Arthur. It mattered that he’s gone. It’s a different world now. And you have to run with that.”
IGN got a bit more out of Whedon in their interview, where he explains that he's making the movies so that they are completely self-contained for viewers who only watch the movies.
“It’s a weird little yes and no. As far as I’m concerned in the films, yes he’s dead. In terms of the narrative of these guys [The Avengers] his loss was very important. When I created the television show, it was sort of on the understanding that this can work and we can do it with integrity, but these Avengers movies are for people to see the Avengers movies and nothing else. And it would neither make sense nor be useful to say ‘Oh and by the way remember me? I died!’”
With Whedon taking a further step away from the MCU, this could change in the future.
I'd like to offer up an in-universe answer if possible - although Joss Whedon may believe that for the purposes of the story he is telling that Agent Coulson is dead, it is a verifiable fact that inside the Marvel Cinematic Universe, Agent Phil Coulson is alive and leading his own version of SHIELD.
In The Avengers/Avengers Assemble, The Avengers were unable to work together effectively as a team for most of the running time - as Bruce Banner stated, they weren't a team, they were a time bomb. Between the clash of ideologies, egos and moral stances, as a group they were simply incapable of teamwork in any real sense, and played right into Loki's hands exactly as he intended.
When Loki kills Agent Coulson, Director of SHIELD Nick Fury lies about the circumstances of his death (specifically, covering his slightly foxed Captain America trading cards in blood) in order to present Agent Coulson as a martyr, a man who died fighting for what he believed in. He does this to spur The Avengers into action - it is Coulson's death that ultimately led to the true formation of Earths Mightiest Heroes, giving them a common goal and a reason to fight in spite of their differences and difficulties.
And Coulson knows this. From his perspective, he is the inspiration for The Avengers, and indirectly the reason that New York wasn't lost to either the Chitauri or the nuclear bomb that the World Security Council sent in during the Battle of New York. Agent Coulson is to The Avengers what Uncle Ben is to Spider-Man - and he isn't going to risk them losing their biggest source of motivation by revealing to them that he is still alive.
Coulson is kept out of the MCU movies for two reasons: He and his team are busy with their own things while the movies are happening(like, for instance, Jiaying's Inhuman uprising, which was going on during Avengers: Age of Ultron) and, as stated by one of the Koenigs(although I don't remember which one) none of the Avengers other than Cap know that he is alive. Fury's statement in Captain America:The Winter Soldier that "they can't kill you if you're already dead" seems to extend to Coulson, in that it is much easier for him to perform his duties as Director of SHIELD if the number of people who know he's alive is kept to a minimum.
