Brown is physically better than Richards right now. He may not be putting up many points, but he's also not playing the way he did last year either. Richards, unfortunately, is playing the way he did for most of last season.
Why send Richards down? Well, like you said, it's not for the cap space. It's the last straw. The Kings have given Richards ample opportunity to get back to being an effective player, even in a 4th line context. They even didn't buy him out. Sending him to the AHL, after waiving him, at this point in his career, now we find out if Richards still has something left, or if he doesn't.
Out of the playoffs, and a close friend is now gone until further notice. Maybe it'll be a wake up call to the rest of the roster, who have up until this point in the season played as inconsistently as a team possibly can.