Because the Americans owned ICANN, they got away with first-level domains for everything, .com, .gov, ... Other countries had to either put everything under one domain, or split it up like the brits.
.com, .net, .gov etc. were the first TLDs created in the early 1980s (when the DNS was invented, before ICANN existed). Only later in the mid 1980s were country codes added to the repertoire, and each country was given one based on the ISO 3166 standard — including the US (.us).
The only domains with geographic prohibitions were .gov and .mil, being run by the US Government. People of all countries have always been able to register domains in .com, .net etc., not just Americans.