Flash: you open the page, it plays. There might be one additional click required depending on your browser settings.
This: you open the page but it needs to be downloaded. You download it. You unzip it. You see that it's a Windows executable but you're on a Mac. You open it hoping that wine will run it. It does. You choose your graphics settings. You click OK. It plays.
Though I'm aware that Unity is capable of running on the web, I haven't seen that capability used much.
