Political and/or financial gain.
It could be the government in question was failing in some way and needed to divert attention.
It could be the government was under financial pressure from a given political group or the leader of that group had a vested interest in war for whatever reason.
Lets say a top politician, I'll call him Mr. Tree, had a family investment in a major arms related company, say, the Vanlyle group.
That company would make $ billions if the politician started a war.
Of course, I'm sure that could never happen in a western country.
You would need the president on board though, and I see nothing that would be worth the risk for him. A family bond isn't going to cut it either. If it worked that way, every bank robbery would involve mothers, brothers, sisters and fathers, and a plot to murder 3000 people is a big step up from robbing a bank.
The fact is, only a psychopath would entertain such a notion, and the odds of hundreds of psychopaths somehow finding each other and being in the right positions to pull off a job like this, is about as close to impossible as you can get. I mean how many fields of expertise would be required for it? You'd need an entire psycho crew of demolitions experts, you'd need psycho trade center employees to cover it up, you'd need psycho politicians, you'd need psycho CIA members, psycho NSA members, psycho clean up crews to hide the explosive materials found in the debris, you'd need an entire 9/11 commision made up of psychos, and a psycho president? It's an impossible scenario.