It could not have passed because the first Medicare act was signed into law by former President Lyndon B. Johnson who did so in 1965. It was the culmination of efforts by various presidents over the years including Present Truman.
One could say that Johnson knew to try again only 3 years after Kennedy did because the passage of the bill under Kennedy had been missed by only a few votes in the Senate. Johnson therefore believed that times had caught up and proceeded to try and succeed.
