Civil War was a seven-issue story in the comics, and even then it went beyond that limited run to have various other tie-ins elsewhere in the comic universe. It was a big, sprawling story that touched upon pretty much every major character at some point or another. Captain America: Civil War, on the other hand, is going to be a (roughly) 140-minute film. You just can't possibly fit everything from the story into that sort of timeframe. There's too much going on, and the film really has to give a lot of focus to the more direct battle between Tony Stark and Captain America, so a lot is going to have to be cut. While this can be for the better, as it will present a more streamlined story, it does mean it can't really be like the comics, due to the fact there will be plenty of stuff they have to leave out purely due to the time constraints and the need to deliver a focused movie.