There are two primary reasons as for why the North was "destined" to win (following the withdrawal of the US combat forces).

1. The dire South Vietnamese supply situation versus the North Vietnamese supply situation.

By 1975, the Ho Chi Minh Trail was not some dirt path cut through the jungle by which porters were still using donkeys and human labor to carry small arms into the South for the insurgency. It had become a two laned, paved highway by which heavy armor and trucks could transit into South Vietnam. In addition to the vehicles and supplies being transported into the South by way of the trail, there was also an oil pipeline that also followed the route. With the lack of heavy American air assets to attack the trail from the air, as well as the inability of the South Vietnamese to successfully interdict traffic on the route, the North Vietnamese were able to stream an amazing amount of forces into the South for their conventional offensive.

Meanwhile, consecutive cuts to South Vietnamese aid by the United States Congress, as well as the strict resupply terms of the Paris Peace Accords, meant that the ARVN were in dire straits. Trained to fight a "rich man's war," the economic recession had left the ARVN without an adequate amount of ammunition, artillery shells, fuel, replacement parts for vehicles or aircraft, and even medical supplies for the combat attrition rate. Even the North Vietnamese were astounded at how much the ARVN were being forced to ration their supplies.

2. The inability of the South Vietnamese (namely President Thieu), to organize a cohesive defensive strategy, starting from the fall of Ban Me Thuot in March of 1975.

With Ban Me Thuot captured, panic sparked among ARVN troops and their dependents (who often followed military units around, causing severe depletions in morale as defeats started to rack up, leading to desertions) as it began to be clear that the strategy of defending the coastal population centers would be rendered moot by flanking PAVN divisions. In general, the defensive situation of South Vietnam was not ideal, as the remote interior portions of the country were difficult to defend, but also sparsely populated. Thieu's decision to withdraw from the Central Highlands, without informing the majority of his commanders ahead of time, dealt such a blow to the ARVN that it was effectively crippled for the rest of the 1975 Spring Offensive. Units and commanders with unclear orders struck their defensive lines as it became clear the original plans to defend Hue and Da Nang were abandoned. Many of the men in these units deserted in order to secure evacuation for their families in and around these cities. The disorderly evacuation down Highway 1 resulted in the deaths of many civilians as the North Vietnamese indiscriminately shelled the column, filled with both families and soldiers alike.

Although some units stood and fought, the evacuation really ended any chances of the South Vietnamese putting up a cohesive defense because of the lack of trust in Thieu, having pulled the rug out from under MR I. In concept it makes sense to abandon indefensible population centers and tighten your southern defensive lines around Saigon and the Mekong Delta, but once you take into account the actual logistics of an operation, and then the idea that it would have to be kept secret from the men until the last possible moment, it really was a terrible decision.

The ARVN's last stand at Xuan Loc proved however, that South Vietnam still had some fight left in it, and the fighting in and around Saigon up until the ceasefire order on April 30 does show that many units of the ARVN were intact and were intending to stand their ground in the capital. Additionally, the largely unmolested Mekong Delta region was home to largely intact ARVN forces centered around Can Tho, and there were preliminary plans drawn up to evacuate remaining forces from Saigon to Can Tho if the city fell.