The nature of combat. Most engagements aren’t above platoon or company size, and many are much smaller, so there is much less opportunity for people to be cut off from their unit and have to surrender.

The nature of the enemy. Less ability to absolutely pound U.S. forces with huge amounts of mortar and artillery fire, which, compared with other circumstances, might lead surrounded units to surrender.

Much lower casualty rates, perhaps depending on the time and place, generally between 5-15 percent of what they were in past wars. Lower casualties mean one [less] factor which can demoralize soldiers under other kinds of pressure.

No air strikes, and hence shootdowns where an aircrew parachutes successfully to the ground….  Most U.S. personnel captured in the Vietnam War were aircrew.