They did write "a key issue" and not "the key issue"...
The US has over 300 million people and growing. The UK only has just over 64 million, but the ccTLD there is huge, and also does not allow non-UK entities to register. So it's not about allowing non-US entities to register, although no doubt that would have resulted in much bigger registration numbers, perhaps even huge.
It's mainly about public awareness, marketing and other factors like a privacy option. If the American public were made to be fully aware of the availability of .US and allowed to have optional privacy, that is what could awaken the "sleeping giant" that has been sleeping since April 2002. To this day I can even speak to a current employee of the US federal government who did not even know that .US exists, which is exactly what I did just months ago with exactly such a friend of mine who has been working with the federal government for many years and is even very close to retiring already. Speak to others here and you can well expect to likely hear something like ".US? What's that?"
And notwithstanding the apparent contradiction of the way people behave with something like Facebook, for instance, which probably has more to do with simply not knowing how to use it for privacy, Americans seem to generally consider privacy important - at least as an available option.
