Why did people have to be drafted for the Vietnam war?

Answer 1

At the time, the Army was not enlisting the large numbers of servicemen needed for a ground war. So, as in World War I, World War II, and the Korean War, conscription began.

Since that time, the higher pay and benefits, plus long periods without major wars, meant that the armed forces were able to reach their recruitment targets. Where they didn't, selected units of the National Guard were called up to combat duty. These units have been more fully trained than at earlier times. Another factor is that war has become more highly automated, with less need for large numbers of combat troops.