There are teams that would take him, but none that would offer anything near his potential value. So the Kings can run the risk of seeing him develop into a star with another team, or keeping him and seeing him continue to lose value as a trade asset.
Have there been any young headcases in the NBA who turned it around while still with their original team? Zach Randolph and JR Smith seem to have had the light bulb come on in different places, and you have cases like Rodman/Artest where they were still goofballs but were put in situations where the actual harmful stuff was minimized. It's very possible that Cousins would just have a better shot turning it around somewhere else (if not for the change of scenery, then for Sacramento's recent track record of developing talent), which just adds to their dilemma.