As was the case, and still is the case, the men on stage (Roosevelt,Truman,Kennedy,Nixon,Bush,Clinton,Reagan,Bush) have nothing to do with it. The men behind the curtain control it all.
In Truman's case, they picked him to run for VP of the US with Roosevelt. (1)Roosevelt didn't like Truman and never would have picked him. Roosevelt knew this was his last time in office, he was diagnosed with a fatal sickness (which he kept quiet from the press, and never had acknowledged himself, but everyone knew) and he was pretty bad off, so when told they were going with TRUMAN, he simply said he really didn't care who they picked.
(2) 'They' had decided on Truman because all other choices had 'little problems'. 'They' had made their decision, and put it into works , before notifying TRUMAN. They got together and had a meeting, where they invited TRUMAN. They had spent several hours beforehand rehearsing a 'phone call' with ROOSEVELT for him to ask TRUMAN to accept.
When TRUMAN got there, the phone rang, Roosevelt gave his rehearsed 'speech' and TRUMAN accepted. ALL controlled by these men 'behind the curtains'.