I think it’s different in each case, and a combination of different things, which don’t add up to a clear picture, and which do indicate less than complete and clear information, such as:
1) TV news media regularly does tell the stories it chooses in the ways it chooses, which often isn’t very close to what’s the complete truth.
2) The Bush administration also did a lot of lying and bending the truth for its own purposes on various points.
3) For example, both the Bush admin and the news media repeatedly said “no one imagined this could happen” when that’s nonsense. The government had anticipated the possibility of such attacks, and there had been attempts to blow up the WTC before.
4) There was a giant wave of “patriotic” behavior by so many people in response to the attacks, that it seemed to be drowning out any rational voices, so there was a sense by others that a lot might be being swept up in that. For example, that’s how we got The Patriot Acts I & II, Guantanamo Bay, Homeland Security, TSA up the wazzoo, justifying the invasion of Iraq by non-existent “weapons of mass destruction”, and all kinds of people using the expression “the world changed on 9/11” as an excuse for all manner of crap, signalling they will not listen to any counter-argument. In that atmosphere, with so much other crap being foisted upon people, it’s reasonable to be skeptical and suspicious.
5) The attacks had some things about them that seemed unexpected, and information and explanations were not forthcoming. e.g.
5a) Buildings next to the Twin Towers that weren’t hit by airplanes, but imploded anyway.
5b) The visible hole in the Pentagon looked much smaller than a jetliner, and no recognizable plane debris was visible.
5c) There were no plane wreck pictures that looked intuitively like a plane wreck for any of them, even for the flight that crashed in the countryside.
6) People on the Internet started building various theories, which involved a bewildering amount of detail, and people arguing on both sides were almost never calm enough to be rational and discuss enough to cut through the weight of all of that.