It's because they are one-dimensional, serving a purpose to the point of her novel. Hank Reardan is an uncompromising entrepreneur who wants to see his Reardan Metal succeed, and is completely uninterested in how his decisions affect anything but himself and his company. Dagny Taggart is only interested in seeing Taggart Transcontinental succeed, especially the Rio Norte line. James Taggart is the contrasting lethargic tumor of a leader who cares more about saving people's feelings rather than making a profit. Even John Galt is rather one-dimensional, trying to create a "strike" of those who make the world work. It works for her novel that they are what they are; not one really is a protagonist in the sense that you'd otherwise see.