Just a thought: LBJ was reluctant to support HHH because of his resvernations with the Vietnam War. Although endorsing him publicly, the president was not satisfied with his second in command. If Jackson had been the nominee, LBJ could have released Nixon's sabotage talks with Thieu and Anna Cenault and the election would have been over.
There probably would've been an indie McCarthy run to roughshod all over anything saving the sabotage talks could've done, and LBJ was still not all that popular.