I'm not trying to argue one way or another on this wall issue, but I don't think you can compare a massive public project (i.e. the wall) with a largely private commercial development (the rebuilding of the World Trade Center).
You also say there was little controversy, but that isn't true either. It took a long time to formulate the scope of the memorial that people could agree on. This had to be done before any other planning could be started in earnest.