Because
A) maybe they don't want to pay him big money
B) Baron Davis, Monta Ellis, Stephen Jackson, Al Harrington and Andris Biedrins all see major minutes. Marco Belinelli is also a perfect player for their system. Brandan Wright should be seeing time too. I have a feeling they might just cheaply resign Matt Barnes to backup Jackson at sf, and start Ellis and Davis in the backcourt.
Basically, for the money he's probably wanting, there's no minutes for Pietrus.
Minnesota already have a younger version of Posey in Corey Brewer
Though I admit a wing-rotation of Posey, Davis, Hassell and Brewer is the best they've had in 3 or 4 years..
They could send one of their pg's to GS as well (though they're all overpriced and injury prone, or just plain crap)