The answer originally was “Because that is where all the oil was – in Saudi Arabia!” America allowed a handful of radical Islamic sheiks to dictate its foreign policy for the whole region.
It did not come up with a foreign policy of its own – especially towards radical Islam – whose policy is simple “Get rid of America!”
America could have said, in response “You have to chose between a tribal past or a global future. We will not force this decision on you – you have to decide for yourself.”
The average person would not have to understand this – they would just listen to what their preachers said. America would have to convince them – that joining America was in their best interest.