To me, it’s primarily easy access to guns. No other factor really comes close, IMO. Another huge reason is we’ve glorified war and solving things via violence for decades and decades — movies, TV, computer games, etc. This, of course, just makes easy access to guns that much more lethal.
No other nation on earth has this combination to such a degree: the society-wide glorification of violence, revenge, vengeance, with the absurdly easy access to guns . . . and now, “open carry” laws which in many states don’t even require licenses, testing, training, etc.
A third reason may return, due to our eroding infrastructure and Trump’s accelerating deregulation: lead contamination. Some researchers believe the recent decline was at least partially due to a reduction in exposure to lead. This may well see a reversal.