Archer discovered location of Kir'Shara after he became host of Surak's katra. But, that's not the first time anyone became host of Surak's katra. Syrran (who transferred katra to Archer before death) was also a host of Surak's katra. Why did Syrran fail to discover location of Kir'Shara? But, a human as a host of Surak's katra discovered it?
To quote roughly 25 minutes into ENT 4x08, Awakening:
(Within Archer's mind)
Surak: You can accomplish what Syrran could not.
Archer: But.. Syrran...
Surak: Was a Vulcan. And you're human. Which means you're untouched by a culture that can no longer see its own imminent destruction. You cannot let what happened to the Vulcans of my time take place again. Find what my people have lost. Find the Kir'Shara.
(Back in the real world)
T'Pau: I failed. Surak chooses to stay where he is.
A few minutes later...
T'Les: He agreed to the ritual
T'Pol: If he hadn't, you would have forced him. You claim that Surak's teachings have been corrupted by the High Command, but you're no different.
T'Les: There is a profound difference.
T'Pol: You deceive yourself.
And a moment later, Surak's memories show Archer where the Kir'Shara is. Surak simply could not trust any modern Vulcan with the information; they were too close to the problem, and it was too easy for them to corrupt the teachings in a different direction. Only an outsider could be trusted to make the knowledge public enough that no one could claim their interpretation was the correct one.
Additionally, the artifact itself was behind a locked door with a very unusual combination to open it (twisting one of the symbols?), covered in dust, behind a passageway filled with ancient tombs... Easy to overlook, as each new arrival would assume someone else has already looked in there. And the door itself was styled in a way that it could have looked like a decoration on a wall, rather than a door.