Final answer:
The Vajont Reservoir Disaster was primarily caused by a massive landslide that led to the overtopping and flooding of downstream villages, with poor site selection and inadequate geological testing as contributing factors.
Step-by-step explanation:
The main reason for the failure of the Vajont Reservoir Disaster in 1963 was a massive landslide that occurred in the area. This landslide displaced a significant amount of water in the dam, causing an overtopping of the structure and a subsequent flood that affected many villages downstream. The death toll from this disaster was close to 2,000 people. Contributing factors to the disaster were poor site selection, lack of thorough geological testing, and ignorance of warning signs such as shifting and fracturing rocks. Furthermore, the area around the dam was known to be prone to large landslides, and the valley's steep canyon walls were composed of limestone with solution cavities, indicating a lack of stability.
Comparatively, other large dams, like the Three Gorges Dam, have been built with stronger safety measures and have thus been able to withstand significant environmental stressors such as massive floods. The safety and economic benefits of properly constructed dams are significant, as seen in the case of the Three Gorges Dam contributing to the economic development of China, despite its own problems and negative impacts on culture and the environment.