There might be a perfectly reasonable explanation for this, but when doubt exists it sometimes takes hold and grows. People see links that may or may not actually be there and things snowball from there.

In the case of 9/11 some conspiracies have little or no substance. Such as the controlled demolition conspiracy. You’d have to remove a considerable amount of material from the building in secret to do that, and you’d need a massive workforce, yet nobody has ever found where all of the material supposedly removed was dumped, and nobody has never identified a single one of the 250-300 men that you’d need to rig such a large building for demolition.

However, other conspiracies are not so hard to dismiss. Such as why Washington is still stonewalling calls for an independent investigation, and is still refusing to grant subpoena power.

If everything were exactly as they said, why won’t they allow people to be pressed under oath?

Speaking more generally, people like having stories with a beginning a middle and an end. And strong narrative. Conspiracies provide them with this, and they are often far more exciting than the truth.

Of course, some conspiracies have been proved to be real. Such as the Feds secretly doping people with mind warping drugs as part of brain washing experiments (MKUltra) or the tuskegee syphilis experiments when the Feds use African Americans in unethical medical experiments. Since the Feds are known for doing stuff like this, it’s perfectly reasonable to suspect them of all kinds of conspiracies.