I've had to deal with a certain phenomenon with conspiracy nuts: When asked for evidence of a conspiracy, they start citing who benefited from the central event. The problem with this, of course, is that benefit does not prove that the person in question planned it. Hindsight is 20/20, but our past selves do not benefit from it. Neither do previous generations.
For one example of the fallacy, many 9/11 twoofers point out how much the Bush administration was able to expand their power and influence after the attack. To someone who thinks governments have a supernatural prescience, this is incriminating evidence. To a more sensible person, this is easily explainable by opportunism, rather than an elaborate conspiracy: They didn't plan it, but they were able to take advantage of it anyway.
People are people, and only have access to certain fields of knowledge. In war history, this is the fog of war: Decision makers don't have instant access to all the relevant information. It could be anything from enemy disinformation to accidentally skipping over a critical bit of a report. Some intelligence agencies prefer not to share information, preventing it from getting to the relevant people in time. This tendency was one of the reasons for the founding of the Central Intelligence Agency in the US. Of course, even with this, government secrets still run the risk of remaining secret to the non-CIA portions of the government.
Large organizations are made up of small parts, namely people, and their means of communication. All too often, conspiracy theorists are unwilling to even think about the difficulties involved in running them. Just because certain information is readily available today does not mean that it was always so. Confusion is, and always has been a part of civilization.