Explaining Divergent Levels of Longevity in High-Income Countries
This is an open access report in book format that delves into the factors that cause regional variations in life expectancy and rates of age-related disease - and thus longevity - in the first world. Given its origins, don't expect to see much on the stifling regulation that raises prices and reduces quality and innovation in medicine, but most of the other important contributing factors are addressed: "Over the past 25 years, life expectancy has been rising in the United States at a slower pace than has been achieved in many other high-income countries. Consequently, the United States has been falling steadily in the world rankings for level of life expectancy, and the gap between the United States and countries with the highest achieved life expectancies has been widening. International comparisons of various measures of self-reported health and biological markers of disease reveal similar patterns of U.S. disadvantage. The relatively poor performance of the United States with respect to achieved life expectancy over the recent past is surprising given that it spends far more on health care than any other nation in the world, both absolutely and as a percentage of gross national product. Motivated by these concerns, the National Institute on Aging requested that the National Research Council convene a panel of leading experts to clarify patterns in the levels and trends in life expectancy across nations, to examine the evidence on competing explanations for the divergent trends, and to identify strategic opportunities for health-related interventions to narrow this gap."
The main reason is pretty obvious, the US is the only western nation that doesn't provide universal health care for its population. US lifespan will never catch up as long as that is the case.
Poor diet and lifestyle also play a role, but from the figures I've read only account for about 10% of healthcare dollars spent in America.