I was also wondering. The reason is that he left Sweden and apparently moved around quite a bit. He is reported to have been residing mostly in Berlin and his girl-friend lived in Gdansk, Poland.

[edit: addition] The reports are ambiguous; it sounds like what they mean is: the Swedish police collaborated with the Polish police to track him down.

Source (in Swedish): http://www.expressen.se/kvallsposten/pirate-bay-grundaren-gr...

Because the copyright mafia pays well.

edit: what? is there actually a question of whether these actions were born of corruption?