Humphrey's favorite-son candidacy in 1952 was really just a favor to Truman, as Humphrey explained in his memoirs, *The Education of a Public Man*:
"As the presidential primaries of 1952 drew near, I found myself in an awkward position. I was devoted to President Truman and had been prepared to support him again, but he had not publicly stated whether he would run or not. Senator Kefauver had already said he would challenge Truman in the primaries, and his friends in Minnesota said he would enter our primary. What happened in Minnesota was of great political concern to me, and I asked Truman's political advisers what they wanted done. They urged me to become a favorite-son candidate as a holding operation until Truman made up his mind. At a minimum, they thought it would keep Kefauver out. It was risky. Truman was not particularly popular, his rating in public opinion polls very low. To be the surrogate for a popular President is not easy. For an unpopular one, it is worse. Voters, and particularly in an independent-minded state like Minnesota, are not comfortable, for the most part, with the gimmickry of stand-ins. They prefer to vote for bona-fide candidates, which I clearly was not.
"If I entered as a favorite son and Kefauver came in, there was at least the chance that I would be beaten, embarrassing me at home and probably in Washington, too. If I were to lose, it would encourage the Republicans to consider me vulnerable in 1954. But Truman had been a good President and good to me. My loyalty overcame my caution and I let my name be entered in the primary. Kefauver stayed out, saying he did not want to cause me any political trouble or embarrassment. I easily won the primary.
"When President Truman announced that he would not run for reelection, it was not unexpected. He had been through exceedingly abrasive years, though I think that alone would not have prompted his decision. It was, I suspect, more a personal decision than a political one. He and Bess Truman had had enough of Washington. Power, even of the presidency, was not that attractive or compelling for either of them. He had done as well as he could, made the decisions he thought were right for the country, and the time had come to move aside..."
https://books.google.com/books?id=L6u94c-7wX8C&pg=PA129
If Humphrey had been a serious candidate, Kefauver would probably have opposed him in the Minnesota primary, and could have caused him some embarrassment (even by coming close).