Why did 9/11 happen when it did???

[Crumpets]

Looking at it from al-Qaeda's perspective, there is a pretty clear answer. It was just the minimum amount of time it took for al-Qaeda to have the necessary means, motive, and opportunity to target the US on that scale after its founding. Al-Qaeda was officially founded in 1988 as the Soviet war in Afghanistan came to a close. It was able to conduct its first international attack in 1992 in Yemen. In its very early years, al-Qaeda leadership (i.e. bin Laden) wasn't particularly interested in targeting the US directly.

However, the Gulf War, and specifically US basing in Saudi Arabia changed that. Bin Laden's extremist rhetoric against the US basing in Saudi Arabia was a major factor in him relocating to Sudan, also in 1992, which I think it's safe to say is really when al-Qaeda's efforts to directly target the US on a grand scale began. Interestingly though, they miscalculated the US's reaction to such an attack, thinking (not too differently from Japan on the eve of Pearl Harbor) that a massive attack on the US would dissuade the US from further activity in the Muslim world, rather than be a catalyst for increased US involvement across the region.

Bin Laden issued his first official fatwa against the US presence in Saudi Arabia in 1996, around the time of his relocation from Sudan to Afghanistan and also around the time Khalid Sheikh Mohammed originally pitched the idea of hijacking commercial airliners and crashing them into twelve different locations across the US. At this point, bin Laden reportedly rejected the idea as being "too elaborate."

In 1997-1998, al-Qaeda made a concerted effort to grow their notoriety in the west, with bin Laden conducting multiple interviews with western outlets and bin Laden and Zawahiri issuing a second, joint fatwa laying out their justification for targeting both American members of the military and civilians. The Looming Tower presents the May 1998 interview with ABC as when the countdown to 9/11 began, or as a "point of no return" for US-al-Qaeda relations. I think this is a bit of dramatic re-interpretation for the series, but it's also grounded in a lot of truth that there was a major inflection in 1998 that made a major confrontation between the US and al-Qaeda seem much more inevitable with hindsight.

Two months after that ABC interview, al-Qaeda conducted the embassy bombings in East Africa. While you can tie al-Qaeda to events like the 1993 World Trade Center Bombing or some of the activity in Somalia in the early 90s, this was the first direct attack by al-Qaeda on a US target. Not at all coincidentally, the attacks took place on the 8th anniversary of the arrival of US troops on Saudi soil. From al-Qaeda's perspective, the attacks were a huge success, as the only US retaliation was a missile attack on some bases in Afghanistan that caused no long-term damage to al-Qaeda, and it seemed clear to them that al-Qaeda could attack the US with impunity until their aims were achieved.

A few months after the successful embassy bombings, in late 1998-early 1999 bin Laden gave Khalid Sheikh Mohammed the go-ahead to proceed with a "scaled-back" hijacking plan, in what would later become the 9/11 plot. This is really the point when the timing becomes more logistical than anything else.

There were two other al-Qaeda plots against the US in the meantime. The failed millennium attacks in January 2000 and the successful attack on the USS Cole in October 2000. Neither received any sort of substantial blowback from the US, in part because of concerns that a heavy-handed response would be seen as a "Wag the Dog" type situation. The limited response to the 1998 attacks had already been heavily criticized because (possibly coincidentally, possibly not) it took place three days after the Lewinsky scandal broke and critics claimed it was just an effort to distract Americans and the media.

Ironically, the scale of devastation on 9/11 probably was limited somewhat due to poor timing from the attackers. The first plane hit early enough that most people with a 9am start to their work day would not be in the office yet, while the fourth plane was hijacked late enough that it was already clear what was happening, the two potential targets had already been evacuated, and the passengers were able to respond even before it reached DC.

If this question is meant to be broader, as in "why did this unique event happen in the early 21st century," I probably can't give as detailed an answer, but the end of the Cold War played a massive part. The transition from bipolarity to unipolarity paved the way for a lot more unilateral action, both from actors on the ground like Saddam Hussein's invasion of Kuwait, from the global hegemon in its responses to those action, and from random other forces who were suddenly not part of proxy wars and had no attention on them as a result. In short, there was an inevitable resettling of the global power balance in the wake of the collapse of the USSR, and one of the few groups that had both the means to conduct major attacks, the motivation to attack the global hegemon, and the opportunity to do so without being caught was al-Qaeda. And even then it took them a full decade to decide it was worth it and actually execute it.