the murder rate is falling for the same reason that the birth rate is falling,
and also why there's fewer bands forming and lower church attendance, why teens are less likely to get drivers licenses or participate in team sports, why brick and mortar retail is in a death gasp, why we're in a "sex recession", why shared living spaces in new apartments are smaller, why volunteering rates are at their lowest in decades...
all of these things require or enable people to be around other people—and that's not happening any more.