The short answer is that Samsung is working to develop a single ecosystem for all their products, including smartphones, tablets, TVs, and home appliances. "So far, Samsung hasn't been sticking to one platform," Samsung Electronics Medial Solution Center President Hong Won-pyo told reporters. "Like Google’s Android platform, Tizen could be a common platform that can connect Samsung products in a single category.”
What with the rise of Android Wear and Android TV, Google's Android platform could probably have served Samsung's needs - if not for the fact that Android is controlled by Google. Samsung officials said without apps and its own ecosystem based on content, there is little chance to go beyond just as a fast-follower rich in manufacturing.
Samsung may have used Android to dominate the non-Apple tablet and smartphone market, but they don't want to let another company control their future - or their pocketbook. This is why they have been maintaining their own app store, and why they continue to invest in Readers Hub and other content stores.