I believe the @breakingnews Twitter account existed well before the app. I imagine NBC News committed to the brand based on its Twitter following before it evolved into an app.

I don't see why editorial responsibilites couldn't be a little more decentralized to lower costs. Give a number of editors Breaking News-level access in their CMS and check a box when a story fits the criteria. But NBC isn't in it for the public good. The app probably simply didn't generate enough revenue to justify the costs.