It appears me to have been a Deep State operation. Humphrey was compromised by being part of the Johnson war efforts but they thought he had enough cover to survive it. As it was the War continued for another 3 or 4 yrs.
I don’t buy LBJ’a explanation - that he was doing it for the good of the country. LBJ was one of the biggest egomaniacs ever to hold that office and one of the most murderous.
Hump was a much weaker leader and would not be able to withstand pressures that LBJ did for four years..
My theory is that it was a simpler time and Johnson didn’t think he could/would win and was tired of the job were he couldn’t control the World like he was able to control the Senate in his old job.
HHH was in no way an improvement over Johnson when it came to controlling the levers of power from behind.
I think he simply saw that he would not win the Dem nomination.