My attempts to persuade others about the WTC explosives theory have lead me to a few conclusions about why denial is such a strong reaction:

1) It is frightening to contemplate that the perpetrators may have been domestic rather than foreign.

2) It is difficult to image how someone could have placed the explosives and gotten away with it. In other words they dismiss the "what", because the "how" has not been explained.

3) Many have been exposed to absurd theories about 9/11 and rightly dismissed them, but now have a bias against anything that contradicts the official story. (less common)

I hope these points are useful. It appears that we are going to have to be persistent while we work to get the truth out.