It all comes down to the work of Ancel Keys. He was the guy that got American soldiers to eat the right rations during the war and did all the research into what diets led to the best outcomes.

In his recommendations to the government (after the President had a heart attack), he could have gone with what folks were eating in Okinawa, but this was just after the war when Americans didn't have a lot of love for Japan. Hence the Mediterranean Diet was the recommendation.

Chris MacAskill is the source of my understanding of this and his Viva Longevity YT channel.