Several reasons: The Fat Man and Little Boy were not thermonuclear weapons. The first thermonuclear weapon fired was “Ivy Mike” in 1952.The Germans surrendered on the 7th May, 1945 (V-E Day being May 7th/8th.) Trinity was 16 July, 1945.The uranium payload for the Little Boy

Several reasons: 

1. The Fat Man and Little Boy were not thermonuclear weapons. The first thermonuclear weapon fired was “Ivy Mike” in 1952.
2. The Germans surrendered on the 7th May, 1945 (V-E Day being May 7th/8th.) Trinity was 16 July, 1945.
3. The uranium payload for the Little Boy was ready 24 July 1945. The rest of the bomb had been finished in early May. The Fat Man was an untested design at this point.