Was this your first polygraph? I never had one, but I heard that passing the first one for a high level ticket virtually always takes several tries.
Did they tell you that you passed the poly?
I think I passed the poly cause if I didn't then they would tell me that instead of saying they selected another candidate and honestly I was so depressed and defeated that I just didn't bother communicating with them after that, 3 weeks of constant waiting and reaching out for updates and then finally getting an 'Unfortunately' email kind of did it for me.
I do not know what the details of the situation were, for example was the offer contingent on additional background checks that you failed or on something else, but if it was related to a US security clearance I would seriously consider filing a request for information -- they are required to tell you their findings.
It would probably be an interesting reading (unless one has a very thin skin) and I would do it just for fun, regardless of the pulled offer. My 2c.