Question

Why was the Tet Offensive considered a turning point in the Vietnam War?

Answer 1

The US conducted a traditional war campaign at first. They were winning under those rules.
The Cong conducted guerrilla warfare. That's not about keeping troops in the field or holding specific pieces of land. Viet Cong fighters would fight and then go back to their village. So the guy selling you pineapple today could be your enemy tomorrow.
Tet was not a victory for VC in terms of physical damage done, soldiers killed, or land taken. (Although they were able to do a lot of damage)
Tet was a coordinated attack at various locations. So the damage was psychological as much as any thing. US thought they were winning. Tet said, "Oh no you're not"