[Lightemup]

But why Iraq? Why not Saudi Arabia, when the oil is there and the vast majority of the 9/11 hijackers were Saudi nationals?

Saudi Arabia is the holiest land in the islamic world, housing Medina and Mecca imagine what would happen if Infidels took over these holy sites?
World War III perhaps?

So it became Iraq, because it borders 6 other islamic countries
(Turkey too, although they are NATO members, they are mostly Islamic and the US counted on using airspace and land to build up and launch through turkey) and so the US is able to "control" access to the Iraqi oil supply and also the Saudi oil supply is kept in check, and the US force acts as a buffer against Iranian geopolitical force projections against saudi oil supply and keeps the Shia in Iran and the Sunnis in Saudi from each other.
And on the other side of Iran is Afghanistan, so its pinched in, and they know it, hence the proxy war the Iranians are fighting in the south of Iraq and the support they give elsewhere in the country to the militias.