Quoting Rohan McLeod (rhn(a)jeack.com.au):
Being curious about the difference between
".us" and " " ; I discovered
"Why choose .US?
[Snip sales fluff. ;-> ]
The DNS namespace grew rather than being planned, is the real
explanation. For historical reasons, we've ended up with two-letter
country-code TLDs (ccTLDs) like au. and us. alongside
three-and-more-letter generic TLDs (gTLDs) such as com., org., net.,
gov., mil., aero., coop., info., mobi., and so on.
http://en.wikipedia.org/wiki/GTLD
http://en.wikipedia.org/wiki/Country_code_top-level_domain
And most recentlly, we have internationalised ccTLDs (IDN ccTLDs)...
http://en.wikipedia.org/wiki/Internationalized_country_code_top-level_domain
...but I digress.
The wide and lasting popularity of the major gTLDs from early on has
made a reorg awkward and thus unlikely. For example, US Federal
government domains _should_ have been grouped under gov.us. but instead
emerged under gov. Likewise, US nonprofits _should_ have been in
org.us. but instead sprouted up in org, and commercial firms should have
been in com.us. but used com. instead.
And now, of course. com. and other primary gTLD namespaces are
popular... because they're popular.
The us. ccTLD is primarily used by USA non-Federal government entities
(mostly cities and counties). Even there, it's often spurned as
undesirable namespace. E.g., my local town (Menlo Park, California)
owns menlo-park.ca.us. but is mostly known by
menlopark.org.
(Please note that I've consistently used, above, the full-qualified form
of domain names, i.e., ending in the period that denotes the namespace
root. The closing dot is often omitted but is technically necessary to
avoid ambiguity, and sometimes is required for correct operation of
software.)