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.