Because what was Kenobi thinking, exiling him ? Hett became a Sith after that. He would have made less damage as a Tusken leader.
I dream of a fic with them; Obi-wan could lose, but be healed by Hett, because he can't bring himself to kill one of the last Jedi, or Hett can lose, and Obi-wan can choose to keep him to help him go back to the Jedi path.