Damien

Before George VI's coronation a visiting US President's attendance would require sailing across the Atlantic, making it quite difficult. Even for George VI, transatlantic flight was in its infancy.

The only two realistic coronations for them to have attended were the last two. The Queen's reign was so long that the idea you could fly to Europe was novel when she was crowned. There isn't much to read into their continuation of the traditional, albeit forced on them due to the distance, of not attending.