Because the Vietnamese NVA and occasionally the VC, in many cases, not all, were fighting away from population centers. The Central Highlands, the DMZ area (Khe Sanh, etc.), the Ia Drang valley, Laos, the Parrots Beak, etc.
Because they had to infiltrate formed bodies of troops down the trail, and because they often tried attacking US and Viet positions on the edges of the controlled areas, they were often good targets in the “open” (actually under trees usually) but away from heavily populated areas.
There was no Arclight on Hue or Cholon, think about that.
The ISIS, when they were fighting the US in 2003-2009, did not do this. They very rarely had formed bodies of troops slugging it out in the countryside vs US or Iraqi forces. They were much more like the purest VietCong, hiding among the people, laying mines, attacking by surprise, and then fading away among the people. Much as one would fantasize over it, its just not reasonable or humane to drop MOABs on Ramadi or Falluja.
How does ISIS become a cohesive military threat unless they assemble their military units somewhere to fight it out?
They have Humvees, military vehicles and tanks they took from the fleeing Iraqis and THAT is what they used to defeat the hapless Iraqi soldiers.
It requires CONCENTRATIONS of forces in key positions to attack, overrun, and then hold that portion of the world you intend to make part of your empire (or caliphate as the case may be ).
“Much as one would fantasize over it, its just not reasonable or humane to drop MOABs on Ramadi or Falluja.”
Care to debate that with Boethius?
Those who support terrorists ARE terrorists.