Here is Max Roser's piece on some of the causes with links to the research:
https://ourworldindata.org/us-life-expectancy-low
The gap isn't as stark if you look at life expectancy at age 60, though it still remains. Some of the causes are that the US has more deaths among younger people.
Max Roser suggests some explanations for the life expectancy gap at Our World in Data.
One is smoking. The US had a higher and earlier peak of cigarette smoking than many other countries. And has had more smoking-related deaths as a result. The silver lining is that we should eventually see fewer deaths over time relative to peer countries given the decline in smoking.
Another is obesity. We have high obesity rates compared to other peer countries. And as a result more obesity-related deaths.
We have more homicides than other peer countries. The good news here is the gap between the US and peer countries is closing as the US homicide rate declines.
We have more opioid-related deaths by a significant margin. This is a consequence of the war on drugs, with opioid deaths especially increasing as more opioid addicts went to the black market in the last decade.
We have more suicides. Globally suicide rates have declined, while in the US they have increased. In particular we are seeing more suicides in older adults in recent years. We have far more firearm suicides than peer countries and firearms are much more effective at successful suicides than other methods.
We have more road accident deaths. There is a significant gap here with peer countries. And many of these deaths are among young people.
Income inequality is worse in the US. While we have higher average incomes than peer countries, the poorest Americans have lower incomes than the poorest in peer countries. Poorer Americans have worse life expectancy than wealthier Americans and this inequality in life expectancy by income is higher in the US than in peer countries.
Income inequality affects child mortality in particular. The US has significantly higher child mortality among poor people compared to peer countries, which has made the US overall have a much higher child mortality.
Healthcare access is worse in the US. The US doesn't have universal health insurance and access issues are particularly significant for poorer Americans. This doesn't affect life expectancy among the elderly as much given Medicare, but it affects life expectancy for younger people.
In general higher healthcare expenditures mean increased life expectancy. But the US is an outlier to this relationship with very high healthcare expenditures, but less growth in life expectancy. This is a complex discussion area. The US spends a lot on end of life care for example.
Max Roser doesn't mention it, but the US has higher maternal mortality than peer countries as well. Particularly so for Black and Hispanic women. And after the Dobbs ruling we likely will see maternal mortality rise even more as women die from unsafe abortions and pregnancies.
Much of this is quite lagging, so I'd be wary of looking for specific year changes. Obesity death rates for example in the chart at the link above flattened in the US during that same period while declining in peer countries. Likewise smoking-related deaths are going to be multi decade lagged from the period of smoking.
But some developments have less lag. Like the opioid deaths that particularly increased after 2010 as well.
I'd imagine any ACA effects will be lagged since for the elderly the changes weren't significant.
The opioid-related deaths chart is especially stark. And you can see the large increase in the last decade especially. If you dig deeper you'll discover this was because more addicts turned to street heroin as legal opioid access was cut off and synthetic opioid abuse controls were put in place. In the last few years this is increasing as well from the rise of fentanyl in the illicit market. Fentanyl-related deaths aren't exclusive to opioid addicts either because fentanyl contamination in other illicit drugs is increasing.
Here you can see the stark child mortality gap for the US. Most of this is driven by a large gap among poor Americans. There isn't much gap between wealthy Americans and wealthy people in peer countries.
