Because the south was firmly defeated by the north (plus the south ran out of aviation and tank fuel & artillery ammunition after the US Congress cut off financial aid to them).
South Vietnam's surrender to North Vietnam is what ended the Vietnam War. The North Vietnamese had the goal of Vietnamese Unification and the surrender of South Vietnam in 1975 allowed them to achieve that objective.
At the South Vietnam Presidential Palace compound in Saigon on April 30, 1975 when North Vietnamese communist forces accepted the surrender of South Vietnam.
North Vietnam defeated the South (or the North conquered the South).
There no longer is a North Vietnam, North Vietnam and South Vietnam were joined together after the Vietnam War.
Air war over North Vietnam. Ground war in South Vietnam; Riverine war in South Vietnam.
Then in North and South Vietnam.
Air war-North Vietnam Ground war-South Vietnam
The North invaded the South and defeated them (conquered them).
North Vietnam=Air War South Vietnam=Ground & Riverine War
North Vietnam started the war against South Vietnam; the US went to the aid of the South.
because it was north Vietnam against south Vietnam
At the end of the war.