Was Ness too much of a hot-head?
Or someone, who too often lets his emotions immediately get the best of him, to be an effective and/or pragmatic leader. Like after Oscar got killed, he storms into the hotel where Capone is staying and tries to bait a confrontation with him. Just forgetting the fact that Ness is by himself and Al Capone has a swarm of bodyguards, and is a "Bad Guy with Good Publicity" (as TV Tropes would say).
And then of course, there's the way that he disposed Frank Nitti after gloating to him about gunning down Malone and boasting that he's going to beat the rap.