There are many reasons for the defeat of the U.S. and of South Vietnam. However, here are some points about your specific questions. The South was actually not more industrialized than the North, and also did not appreciably better technology. The North Vietnamese undertook a major industrialization effort after winning against the French in 1954, and were supplied with advanced weapons by the Soviet Union and by China throughout the war. The South never had a comparably strong enough government to shift from an agricultural to industrial economy. A specific challenge faced by the South Vietnamese was that their military was especially politicized compared to the Northern military, and so political leaders promoted allies for their loyalty rather than for their military ability, the best military units were always kept to help prevent coup attempts rather than being sent to fight the enemy, and training efforts were usually small scale and did not enable the units to develop the ability to effectively work together. As a result, when faced with the regular North Vietnamese military, the Southern military fell apart in rapid fashion and was easily defeated in 1975. For more on this, see Professor Caitlin Talmadge's book "The Dictator's Army: Battlefield Effectiveness in Authoritarian Regimes."