I'm guessing Lyndon Johnson was ready to leave the presidency and had enough. 1966, was a really rough year for President Johnson and the Johnson Presidency. House Democrats, lose over forty seats in the House and three in the Senate. Because of the Vietnam War and how unpopular it was. Plus with the Republican Party and House Republicans, finally figuring out how to campaign and win in the South. Richard Nixon of course was a huge help there and he campaigned for a lot of House Republicans and House Republican candidates and did that in the South. Which helped him with his 1968 presidential campaign with all of these new Freshman Republican Representatives, who now owed him favors.
1967, the war gets even worst for the Johnson Administration in Vietnam. And now he's having a lot of problems with his own party in Congress. Starting with Senate Democrats holding Vietnam War hearings that started in 1965, but continued through the next Congress in 1967. Several Senate Democrats are now weighing presidential bids against their party leader in President Johnson. Like Robert Kennedy, Eugene McCarthy and George McGovern. The anti-war movement and the New-Left emerges in the late 1960s. And they're against President Johnson and calling him a war criminal, and war monger and a few things that you can't say on network TV, even today.
1968, was the topper with LBJ now being the most unpopular elected politician in America. And now there's talk if he can even win the Democratic nomination for president, let alone with the presidency against Richard Nixon, Nelson Rockefeller, or George Romney, in the Republican Party. By March 1968, President Johnson decides that he's had enough and doesn't want to run for reelection and announces that to the country. But 1968 is just getting started with two great political leaders, Senator Robert Kennedy, and Dr. Martin King, both being assassinated. There racial riots in Chicago, Detroit and Los Angeles. The Democratic Party, is now divided between their mainstream Progressives, which LBJ represents and their Far-Left. LBJ, was really smart not to run for reelection in 1968.