Ooh, ooh, is it because it would be mindnumbingly stupid?
[reads article]
Yep, got it in one!
Americans have been sold an image of the US being an omnipotent presence, due to its Navy. It is a legitimate question to wonder why a relatively weak, long embargoed country has the power to control the waters when the US has spent a pretty penny on all these warplanes and aircraft carriers.
If little Iran can prevent the US from being able to establish security in a little straight, it (ideally) shatters that image and causes some soul searching for what US taxpayers are buying with the military.
to quote: "in the Persian Gulf today, the Navy grasps the reality of the circumstances, recognizing that it simply can’t sail into the strait without risk getting blown to smithereens by Iran’s missiles. Today, its carriers are stationed well outside the Gulf and the ranges of Iranian missiles."