I mean, pick your incentive:

• The Saudis had been absolutely salivating at the idea of US boots-on-the-ground for decades
• Over the years, multiple scholars expressed the view that the Iraq War was conducted to re-assert the dollar hegemony in the wake of Saddam Hussein's attempts to switch from petrodollar in the oil trade and to sell Iraqi oil in exchange for other currencies or commodities
• Who was the dude in the Bush administration that outright admitted that without the Iraq War, they wouldn't have had the support to pass most of their agenda (specifically the Bush Tax cuts)?
• The entire republican looting strategy revolves around profiting from disaster

I think that the 'safest' normie interpretation is that our long history of overthrowing democracies and funding terrorist groups (i.e. the CIA and the Mujahideen) came back to bite us in the ass.

The more complete explanation is that we never stopped meddling in the Middle East and even if the US wasn't involved directly, 9/11 was either orchestrated with the explicit blessing and financial backing of the Saudis, or that and things were simply 'allowed to happen' with the Saudi's implicit blessing and financial backing, much in the same way that the CIA would back a coup.