It wasn't that simple. Reveling what happened with Njobu would of opened up another issue. Because at that point T'Chaka Tchalla and Zuri knew what happened, and the council would question the validity of "Njobu was a traitor who worked with Klaw, however king T'Chaka issued Justice to him but no one else knows what happened except for Zuri band he omitted that true with a lie that he simply disappeared" that would be hard to pass off, especially when trying to decline Erik's bloodright (Njobus betrayal wouldn't mean anything to Erik in that sense) while he also brought Klaw. It would of been a whole cluster of a mess. And denying the challenge would of out Tchalla at odds with the tribes. He really didn't have a choice. And if he agreed to the challenge and then turned around and had Erik killed?? Yeah he would of out a target on himself and the Royal family because that would of been sketchy as hell