Why did Ngo Dinh Diem end up losing a tremendous amount of

support despite US efforts to financially assist South Vietnam?

Because his government was marked by scandal that resulted in a deficient political and economic situation in the country, in addition to having established a strong religious and political persecution.

Explanation:

Jean-Baptiste Ngô Đình Diệm was the first president of South Vietnam and had strong financial support from the USA, which could have created a strong economy and political stability that was very beneficial for South Vietnam. However, his government was marked by much corruption and scandal that led South Vietnam into a political crisis and a totally weak and unleashed economy that left the population very unhappy and against the government of Ngô Đình Diệm. In addition, he established a totally authoritarian government, full of nepotism and with a strong and violent political and religious persecution, which resulted in large protests that caused him to be deposed and murdered by the country's military