First, the terrorists only have to keep the plot secret up to the time of the attack. The conspirators have to keep the plot secret for (at a minimum) the rest of their lives. Additional attacks necessitate additional conspirators and additional evidence, either of which could give away the conspiracy. This risk is much worse if "they" have to use a missile or something else besides a 757 to attack the Pentagon.

Second, the consequences of discovery for the conspirators are much worse. Should the terrorists be discovered, the likely worst result would be failure of that particular operation (or at least that part of it) and the loss of several operatives who were obviously expendable anyway. Should the conspiracy be discovered, the conspirators will all face lengthy prison terms or death sentences, plus massive damage to their political agenda.

So, multiply the much worse potential consequences by the much lengthier period of secrecy required, and the conspirators' incremental risk becomes quite large, and clearly the small incremental gain is not worth it.

The Pentagon hit by an inside element makes perfect sense - if you consider it as a military operation. That allows you the benefits of essentially making yourself a target of your own attack. Why do that? It instantly appears to make the military one of the victim, and makes the idea that they actually did it such a far-fetched notion as to be prepostorous. "How can you have the nerve to accuse us? We're the victims here!"

- The Pentagon was hit on the only side where renovations of structural reinforcement had just been done. The section was far less populated with workers as a result.

- They can control the target point and scope of damage with more precision using a missile instead of a large, cumbersome 757.

- They can immediately control any incriminating evidence because it's all within their vicinity...

Visual Evidence

We know for a fact that all the outside perimeter videos and photos were confiscated within minutes after the hit. Over 5 years later, none of them have been released. And how many video cameras would have been in normal day-to-day operation, in and around the Pentagon itself on 9/11? They are obviously claiming that only one caught anything on tape - the few frames they have released that essentially show nothing.

Physical evidence

- First hand reports in the immediate aftermath said there was virtually nothing on site that indicated a 757 had crashed there - no wings, no tail section, no seats, no luggage, no engines, no passengers. Nothing but small scraps of metal which were of unidentifiable origin.

- Has the FAA / NTSB done a mandatory accident investigation? Have they re-constructed the plane in a hangar? If they have, I haven't seen or heard of it.

Eyewitnesses

Again, taking it as being a military operation, there is the distinct advantage of being able to support your claim with planted witnesses from within your ranks. Despite this, the Pentagon witnesses still gave widely varying and often conflicting accounts of what hit the building.

http://www.davesweb.cnchost.com/nwsltr68.html

There has, through confiscation and withholding, a thoroughly effective restriction of the strongest and most reliable evidence (physical and visual) from the public.