Part of that is plain old-fashioned colonial thought patterns: the "primitive" Houthis couldn't pull this off. In fact, the Houthis have been improving their drone and missile targeting for several years and have demonstrated considerable skill with the emerging technology.
The US -- and, for that matter, the Saudis -- have enormous firepower, but the possible consequences of such a response are simply too costly. If 18 drones and seven cruise missiles did this much damage, how much could hundreds do? World oil prices have already jumped 20 percent, how high would they go if there were more successful attacks?
The only way to take out all the missiles and drones would be a ground attack and occupation. And who is going to do that? The United Arab Emirates (UAE) has already begun withdrawing its troops from Yemen and has been holding talks with the Houthis since July, (which is why UAE oil facilities were not attacked this time around). The Saudi army is designed for keeping internal order, especially among Shiites in its Eastern provinces and Bahrain. The princes in Riyadh are far too paranoid about the possibility of a coup to build a regular army.
The US? Going into an election with prices already rising at the pump? In any case, the US military wants nothing to do with another war in the Middle East, not, mind you, because they have suddenly become sensible, but as Gen. Joseph F. Dunford Jr., chair of the Joints Chiefs of Staff put it, it drains resources from confronting China.
Starting with the administration of George W. Bush, and accelerated during the Obama presidency's "Asia Pivot," the U.S. military has been preparing for a confrontation with China in the South and/or East China Sea. The Pentagon also has plans to face off Russia in the Baltic.
One suspects that the generals made it clear that, while they can blow up a lot of Iranians, a shooting war would not be cost free. US Patriot missiles can't defend our allies' oil fields (or American bases in the region) and while the anti-missile capabilities on some US naval ships are pretty good, not on all of them are armed with effective systems like the Sea Sparrow. Americans would be coming home in boxes just as the fall election campaign kicked into high gear.
Whether the military got that message through to the Oval Office is not clear, but Trump's dialing down of his rhetoric over Iran suggests it may have.
What happens now? The White House has clearly ruled out a military response in the short run. Trump's speech at the UN focused on attacking globalism and international cooperation, not Iran. But the standoff is likely to continue unless the Americans are willing to relax some of their "maximum pressure" sanctions as a prelude to a diplomatic solution.