Fenixp: Because it doesn't have strange world in any way. In fact, it has one of the most familiar worlds out of all fantasy RPGs.
Geralt_of_Rivia: I disagree. A game world that borrows heavily from Slavic mythology is only familiar to people who live in Eastern Europe. Or to people who read the Witcher books.
It is quite far removed from classic western fantasy tropes, IMHO.
I'm from western Europe (France), with no real contact with eastern mythology, and I found the Witcher to be pretty classic fantasy. Good classic fantasy, mind you, that makes good use of the standard elements, but not really "exotic"