“In state prisons, the mortality rate was 275 for every 100,000 people, the highest since data collection began in 2001.”
That’s about the same as the mortality rate for men 35 to 45 years old in the general population:
And the prison population trends older than the general population:
I’d say the large number of deaths in US prisons is mostly because there’s lots of people in US prisons. Sure, that incarceration rate is a problem that should be solved. But the death rate doesn’t seem out of the ordinary at all.
A fuller explanation here.