That was fast.
And the answer is because of the dreaded possibility of websites like this, for example, which are few and far between now but could be far more abundant and far more popular, widely known, and most importantly far more
INFLUENTIAL to the American public if .US was more widely known and popular with the American public, who now still scarcely know .us even exists at all:
www.Represent.us
The implications of such things catching on in the US are so enormous and so unacceptable to those who wish to continue the way things are normally done in terms of preserving power and control - i.e. what is normally referred to as "the establishment" - that a TLD like .US simply has to be held down like an eagle tied to a post, otherwise unpredictable and disruptive things might happen. Just as have already come close to happening now with the current election season about to reveal its big result tomorrow.
So multiply that example site above times 1,000 or 100,000, and imagine a country in which the American public actually knows that .US even exists, and the risk of real change is no doubt unpalatable in the extreme to various so-called "powers that be" who have authority over those who dole out the contract, etc., all the way up the chain, who then tell Neustar what to do what not to do in whatever form they do that.