For whatever reason, television networks, both broadcast and cable, will occasionally greenlight shows and then cancel them before they ever air. There are a variety of reasons this happens - and as the cost of making TV increases, it's happening more and more.
Sometimes a show is cancelled without airing simply because it's not very good, or at least the executives at that particular network don't think it's coming together well. Or maybe it's because someone higher up the corporate ladder had second thoughts about the show. In a few cases, outside issues that have nothing to do with the show's quality intervene, such as a writer's strike, the death of talent, or contractual disputes.
Here are the many canceled TV shows that were bought, written, and in most cases shot and announced as coming out soon - but never actually made it on the air.