Because only gunboard fanboys are concerned with what kind of handgun grunts carry. Grunts carry what they're told, whatever they might prefer, and wish it weighed less. Despite whatever anyone writes about it, the need to use a handgun in modern combat is pretty rare, and when the military has one that works, at a cost they can get in their budgets, it'll be issued for a long time. Maybe as long as the .45 was. The M9 is already more than a third of the way to that mark.
They still use B52s. Why not use Berettas into the late 21st century?