'Cause it's Polish.
I mean, I could list a whole bunch of other reasons like it's a believable medieval fantasy world I love to explore (by reading and playing), I'm interested in mythology and especially in slavic mythology and I'm grown to love the characters, I wouldn't miss them. It's language practice for me, in different ways, while doing something pleasant instead of cramming grammar and stuff.
But above all, it's the way I came to the Witcher series, and how it's connected with specific memories. Call me sentimental, but in the same year I was brought to the Witcher series by a proud Polish friend, I discovered lots of other great things, I learned how to deal with myself, was taught a foreign language and lifestyle and met most of my current close friends. It was a great and a tough time, and I had moments when I didn't know why I was doing all these things (throughout that year as well as after).
All in all, the whole Witcher thing kind of saved me when it was hardest (after coming back), among with other stuff. It was a world I could immerse myself in while remembering, and additionally forget about other things. It keeped me interested and busy. And it taught me a way to see the course of life: Something ends, something begins.