Andrew Johnson as VP

[AussieMick]

D.H. Donald's "Lincoln" pages 505-506 give some suggestions as to why Johnson was preferred to Hamlin.
Its said that Lincoln regarded the office of VP being unlikely to be involved in executive decision-making. He apparently wanted to , or at least appear to , allow the Convention to make its own choice of VP without any interference from him. Its suggested that Charles Sumner (leader of the anti-slavery forces in Massachusetts and a leader of the Radical Republicans in the U.S. Senate) used influence to oppose Hamlin.

I cannot find any solid reasons why Hamlin had lost favour. He was just as anti-slavery as Sumner. His background was as a Democrat. He was very much opposed to drunkeness ... this tends to make for unpopularity with those who like to drink. "As a temperance man, Hamlin determined to banish liquor from the Senate chamber and committee rooms. The combination of his rule outlawing the sale of liquor in the Senate restaurant and the departure of the hard-drinking southern senators after secession sobered the institution. "
https://www.senate.gov/artandhistory/his...Hamlin.htm

He had swarthy features and there were some that said he was a Yankee with 'black' blood.
I'm sure its been noted elsewhere on this forum that Lincoln once said that he had no fear that the Confederates would assassinate him, because they knew that Hamlin would take his place.