The blockchain is still decentralized in a sense that you can always create your own frontend and decide how to display the content if another one has gone evil. It would just take more time to create. If it was open source it would be a lot easier to just clone it and remove the blacklist but some people don't want others to easily copy code that they spent time, money and effort to create.
Another issue with open source is that if people know how the algorithm works, they will just game the system instead of focus on creating good content and organic growth.