In Greek Mythology, Teiresias was a prophet and a clairvoyant.
Like other oracles, the way that Tiresias obtained his information changed from time to time. Sometimes he would receive visions; other times he would listen for the songs of birds, or even ask for a description of visions and pictures appearing within the smoke of burnt offerings and interpret them.
There are different stories about how he became blind. The most direct: He was blinded by the gods as a penalty for revealing their secrets.
In the end, the Great Seer is blind and that, "The Blind Great Seer" is the irony itself. The correct answer is:
because he is blind
