It's also driven by ignorance. Think about it, the average person doesn't dive far enough in to the war, to get to minute details about every General, or Commander. It would take a person a lifetime to study every notable figure of the war in depth, if it's even possible. This is true even for some folks who fancy themselves Civil War Buffs.
Once most folks settle on their favorite person, or facet of the War, it's difficult to devote much time, or even interest, to other figures, or events. Most folks seem to have their "area of expertise" so to speak. It then becomes easy to defend your favorites, without full knowledge of others, so to speak.