Real politik.
Saudi Arabia is a useful ally to have in the region, a fellow enemy of Russian ally Iran, a good customer for the defense industry, and an alliance makes the US less reliant on Israel. That's a win-win.
Elements in the Saudi government may have helped orchestrate 9/11, but after 2003 attacks in Saudi Arabia they drastically and often violently curtailed the influence of the Wahhabi establishment. The aristocracy clamped down on the power of the clergy. They also offered significant and valuable intelligence cooperation with the US
So if elements in the Saudi government did help orchestrate 9/11, it's easier to sweap it under the rug. There are more important issues to worry about.
Twenty years ago? Other decisions should have been made and not made.
Now? The world and the regime in Saudi Arabia has changed.