If people that are high enough level to raise dead and cast greater restoration have interacted with this poison, how come nobody has used contact other plane, legend lore or divination to find out what the cure/secret focus is?
Thanks for the question.
The secret focus has been discovered before, and lost and rediscovered several times; the history of the Tenesia setting is long (elves, dwarves, and the other immortal races have been around long enough to see apes evolve into humans, and lizards into goblins).
The Darve, as well as a few orders of assassins, have a vested interest in keeping the secret. Discovering the cure would be big news, and it would immediately make the discoverer, her family, her friends, and her home a target. Plus, unscrupulous nobility who aren't beyond a little bit of murder to advance their station don't want the secret to get out either (though they do want it for themselves).
Also, naricium is not something most people experience. As I mentioned before, the Darve are the only ones who know how to make weapons from it. Others who use it do so as a powdered poison, or manage to make a watered-down liquid version (lower save DC). Only the elves, dwarves and goblins who frequently come into conflict with the Darve are likely to ever see naricium, and even then it's not as if all Darve are armed with it (it is quite expensive, as I mentioned). To all other races, naricium is essentially a thing of legend, and some people don't believe it exists: they may claim Darve magic is what kills the victims, or that it's just a myth told to frighten people.
So, the general demand for a cure is fairly low compared to curing many more commonly seen poisons and diseases.