The communists were fighting on home soil. They knew the conditions far better.
The US military was used to the open battlefields of the Korean conflict and the European battlefields of World War II. When confronted by the dense jungle terrain they were hopelessly ill-equipped to handle the battlefield conditions and engage the enemy. By contrast, the Vietnamese communists (Vietcong), the military forces of the Northern Vietnamese National Liberation Front (NLF) army, used their deep knowledge of the local geography to gain the upper hand on the battlefield.
The Vietnamese communists used Northern Vietnamese troops on the battlefields of South Vietnam for large-scale attacks. However, they employed Regional units to engage the American and South Vietnamese government forces on a daily basis. These Regional units operating within their home districts and had a deep understanding of the local terrain. The Vietcong had accumulated skills from a lifetime of jungle life. They tracked American soldiers from their cigarette butts left behind. The US forces found the dense vegetation a persistent problem. The new M16s the US forces used in Vietnam frequently jammed in muddy conditions, making them poorly suited to the Vietnamese battlefields. Disease-bearing animals and mosquitos wreaked havoc on the American troops. The communist forces, growing up in the damp and humid Vietnamese villages were less vulnerable to local diseases. The American forces attempted to level the playing field by dropping Agent Orange, a powerful herbicide, across swathes of the Vietnamese jungle. However, it was not enough to guarantee their victory. The scars of the destruction of Agent Orange litter the Vietnamese landscape and population today (Agent Orange has been traced to abnormally high rates of birth defects, cancer, heart disease, and Parkinson’s disease among the Vietnamese population today).
While the terrain made fighting difficult, the US administration could have done more to overcome the challenge. The individual rotation policy  meant that platoon leaders were pulled off the battlefield after six months. As soon as commanders were beginning to become accustomed to the conditions, they found themselves removed from combat. The policy stemmed from the prediction in Washington that Vietnam would be a short war, when, in reality, it turned into the nation’s longest. The conditions would have been less of a problem had the leader’s in Washington applied basic military logic to their Vietnam strategy. The Individual Rotation policy made no sense, especially for those that lived through it. Major Richard A. Gabriel and Lt. Col. Paul Savage accused the Individual Rotation policy as foreclosing “the possibility of developing a sense of unit integrity and responsibility” and ensuring “a continuing supply of low-quality, inexperienced officers”.
Fighting a war in your backyard, on streets and terrain you know like the back of your hand, gives you a considerable advantage over a foreign enemy.
Enter the technical rejections of the premises here ...