That is a dirty not so secret. Saudia Arabian people were the primary terrorists. Bin Laden was a Saudi. We couldn’t go after Saudia Arabia because they are ostentatiously our “friends”...or at least our corporate friends. So nobody dared or dares to say the truth.
We couldn't go after Saudi Arabia because they threatened to blow up their oil production capacity if we tried anything. In that regard, I think the Iraq War was an effort to flank Saudi Arabia economically, because removal of Saddam meant that Iraq could develop it's significant reserves as a counterbalance to Saudi petropower.
In 2001 the economy was still suffering from the aftermath of the popped tech bubble. Saudi Arabia was responsible for a huge part of world oil production. The US fracking boom was years away. If we'd gone after the Saudis they would have sabotaged their oil fields and there would have been a global depression.
Now that the world is less dependent on Saudi oil it's time to stop putting up with their sponsorship of terror. We don't need to invade them, but in no way should they be considered an "ally."