Why is Civil War a "Captain America" movie?

Question

This question is going to be rife for people giving opinions, so let me remind everyone to support your answer with evidence. In this case, quotes from the producing staff would be strongest if possible.

Captain America: Civil War relates to

an internal schism between factions of the Avengers and other mutates over oversight for the Avengers and other superhumans.

It's true that

a significant plot point in the film relates to the backstory for Captain America with Bucky Barnes

but on the other hand, the plot involves pretty much every living MCU character previously seen except

Thor and Hulk.

If I had walked into the theatre without knowing the title, I would have thought it should be called Avengers: Civil War. I can easily argue that Iron-Man is nearly as important to the plot as Captain America, and the ensemble cast is in full force

So, suffice it to say, the degree to which is it primarily an Avengers or a Captain America movie is debatable. Let's not discuss that because there are arguments on either side.

I can also argue many people would be more likely to go to the theatre for an Avengers movie rather than a Captain America one. On the other hand, the last Avengers movie was last summer. So there are presumably business reasons in play too.

So:

What are the out-of-universe reasons that Captain America: Civil War is titled as a Captain America film rather than an Avengers one?

Answer 1

Kevin Feige addressed this in an interview with IGN

He explains that while there are a lot of characters who show up, they don't get full story arcs.

"There were a lot of characters in The Winter Soldier, but it felt like a very singular and relatively simple thriller. Civil War follows in that same way," Feige said. "I think that's something [directors Joe and Anthony Russo] pride themselves on, and our screenwriters Chris Markus and Steve McFeely are excellent at giving each character just enough. They're not full arcs for everybody; it's just enough that their presence is felt and important, but that the very clear single story that is being told is being served at all times."

The film is ultimately about Captain America proving Bucky's innocence. The Civil War is just a plot device to that end. We get lots of guest appearances, but none of them have much effect on the real heart of the story. You'll note that the final battle involves just Cap, Bucky, and Tony.

Even Anthony Mackie discussed this with MTV recently

That’s what I thought at first, when I first read it, but not at all. It really plays well with the storyline of Cap, and how he’s evolved into his own man, him doing his own thing, everybody coming together to fight for the common good of man.

Answer 2

This interview has quite a few quotes talking about this very topic, but here are a few relevant quotes:

And we always think it’s interesting to push Captain America up against the wall. Something we found in Cap 2, frankly, is we felt Cap was more interesting when he had something to push up against. In that case it was S.H.I.E.L.D. that turned out to be corrupt. In this case, it’s the world saying, “This is how The Avengers can be run.” Who better to push up against that kind of pressure than the guy essentially wearing an American flag?

So in trying to tell the best Captain America story, it turned out Civil War actually gave us sort of the impetus for putting him, once again, at odds with the world, which is where I think he’s most effective.

-- Nate Moore, producer

Bringing it full circle is really important. We’re taking Cap to a place, there’s a level of detail that we have to be careful with, but we’re taking Cap to a place in this movie that he’s never gone before. That for us is taking Cap full circle. How do you take this guy that began where he began and had that great arc that he’s had and still take him to a place he’s never gone before? We always talk about him, he’s such a tough character in a lot of ways because he’s so strong and so centered, he has such strong ethics and morals, how do you upend a character like that? It’s easier to upend a character like Tony Stark in some ways because he’s a little all over the place and balanced and blah blah blah. You can spin him out easier so to speak. So how do you spin Cap out? We found a way to really get at the heart of who Cap is to shake his foundation, push him somewhere I think that’s going to surprise a lot of people.

-- Anthony Russo, co-director

And possibly the most basic answer, because it's simply from Cap's point of view:

It’s all storytelling metrics, and you have to really think hard about those metrics. I’ll say this, obviously it will be easier for the audience to get behind Cap because it’s his movie, it’s his point of view and he has the most screen time.

-- Joe Russo, co-director