Sherk's been injured...and once he returns from rehabbing the injury, he's slated to fight Hermes Franca.
BJ & Jens had nothing lined up, in terms of upcoming fights in the UFC.
They have a long-standing fued.
It adds to the "drama" and "reality" of the show to have a coach on there, who has been defeated by one of his pupils. People will tune in to see..."what could Jens possibly teach Joe?"
That's why it was Pulver instead of Sherk.
i'm a monster
i'm a maven
i know this world is changin'
never gave in
never gave up
i'm the only thing i'm afraid of