Using existing IP is a lot safer, and will generate more sales, even if otherwise the games are the same. There are a lot of people who played Monkey island in their youth and are nostalgic for it, but who don't follow the latest game news and wouldn't auto buy a game without the IP. Yes it could be successful, but that the Monkey Island name it would be more so.
See for example the success of every new Civ game compared to the success of other strategy games.