By his own choice. He has turned down offers. He prefers to play the racial martyr than play in the NFL.
EDIT:
The NFL went to the trouble of setting up a workout expressly for his benefit, complete with press coverage. And he boycotted the event without warning the NFL that he wouldn't be there. He's a flake with a history of putting his personal ambitions over the team's and deliberately making himself a lightning rod.
70% of the players in the NFL are black. If you're going to try to convince me that the NFL has singled out Colon Kaepernick to exercise racial discrimination against, you'd best bring your lunch because it will take you quite a while.
