How was the Battle of Tet a loss?

In the Battle of Tet, Northern Vietcong decided to attack Southern Vietnamese cities. We not only successfully defended these cities from invasion, but killed the Vietcong's' most elite troops in the process. Now I know this was the first battle we ever fought where we were at the receiving end of Guerrilla Warfare, but in every single rule of battle, if you are able to eliminate the enemies elite soldiers, while at the same time successfully defending the place they were attacking, that is considered a major victory. Instead Tet is considered a loss.