Well, I would confer it to a combination of intel dropping the ball, treachery and dissent among certain members, and the aggressors' extraordinary genetic makeup. Their incredible DNA manifested as extreme proficiency for a wide variety of weapons, as well as herculean endurance and the intelligence of Einstein. The universe just seems to have it out for us.....
Oh, and to answer the poll, I chose Autumn and "God Bless the Enclave." Both Eden and Richardson were grand leaders, but they really need to get off this "Kill everyone in the wasteland but us" mindset. One, it hasn't exactly been helping them, and 2, It's gonna take a helluva long time to repopulate just America with only the Enclave's small population and maybe some vaults. Autumn's plan would have worked best: Distribute fresh water to the masses, earning their respect and admiration. With the masses behind us, there would be few states that could challenge us. Not only would they be a valuable military force (think of the Sepoys in India), but would also provide much needed workers.
Edit: Oh and a hearty welcome to you new guy.