a) Saudi nationals have invested a lot of money into the US economy. Either through sitting in American banks, stocks or via the oil supplies which the US is dependant on. It would be unwise to attack a country that keeps your economy going.
b) Saudi Arabia is in effect "pro-american" although I mean that very liberally. It is not as hostile to the US as many other Middle East nations, and in an area where many people would like to see you dead, it doesnt hurt to have a friend.
c) Although the 9/11 men were Saudi nationals, they were Al-Qaeda trained Taliban members whose dictatorship in Afghanistan would be a better target to attempt to disband. Al-Qaedas HQ in Afghanistan would be a more appropriate target. Whereas Saudi Arabia had once supplied the Taliban with equipment and aid to its people, and was one of the few countries to recognise the organisation in power, in Nov 2001 their recognition and aid stopped.
d) Saudi Arabias relations with Iraq during the first/second gulf war was called into question and it turns out that they werent too pleased with Saddam Hussein either - in laymans terms. They also had diplomatic strains with the people who didnt support the Gulf Wars, so again they're allied to the West. They donated $55 billion to the rebuilding of the region and supplied coalition troops with food, fuel, tents etc during the First Gulf War.