I think you are wrong on the favors thing but am too lazy to look it up.
Why would Jabari block any trade to a team that would play him? He’s out of the rotation... if he stays there they aren’t picking it up and his value will continue to be nothing if he doesn’t play. If he threatened to block it any team will be completely out... I’m not sure he’s the brightest most self aware guy but I think he’s be smart enough to connect those dots.
This is Jazz' page: http://www.basketballinsiders.com/utah-jazz-team-salary/
This is Bulls' page: http://www.basketballinsiders.com/chicago-bulls-team-salary/
Favors' contract is a non-guaranteed contract in the second year.
Jabari's contract is a team option contract in the second year.
On your question - in short - because he thinks the Jazz(or some team in the league) will be willing to do it if it came down to it. NOP didn't want to guarantee Mirotic' contract either at the beginning... but at the end they agreed. No team wants to guarantee 20M a year if they don't have to... and no player/agent wants to give up 20M guaranteed if they don't have to. At the end it will come down to who blinks first. Jabari is in a weaker position than Mirotic was last year, but he can still play leverage game with a team if that team is willing to give up value for him.