True open world single-player fantasty role-playing game with high quality is a rare thing.
I just know of one game and that is The Elder Scrolls and that's that, at least the only one I think is good.
Therefore, I think it's easy for The Witcher 3 to capture the attention of new players (like me), that is not familiar with The Witcher. And that's why I think this game will sell a lot more copies, because it's an open world fantasty role-playing game, the freedom to go anywhere you want. There is not enough games like that around in a fantasty setting.