Was he even invited?
Coronation isn’t that common so we can’t call it tradition, but the absence of American presidents goes all the way back to American independence.
Until Queen Victoria, the relationship between America and Britain was somewhat antagonistic. Irish American republicans had additional reasons not to celebrate coronations in person. Plus, journeys across the Atlantic weren’t without risk.
The advent of flight and the thawing of relations made attendance more possible.
Dwight D. Eisenhower was formally invited to Queen Elizabeth’s coronation but chose not to didn’t attend, setting the formal precedent in the modern age.
US President gets invited, and declines. That’s a good tradition I think... Confirms the friendship between the countries while celebrating the right of an American president to say no to the monarch.