It's a sign of stability. The US was recently (in linguistic timescales) formed. There was also a lot of migration within it. The population of UK cities may ha

It's a sign of stability. The US was recently (in linguistic timescales) formed. There was also a lot of migration within it. The population of UK cities may have been (largely) stayed put for centuries. Jared Diamond uses this a lot, when trying to unearth the history of populations. This isn't a new thing - the 1956 Kurgan hypothesis suggests that PIE (the language which everything from English to Indian languages descended from) is a sign of a large empire, or a dominant culture which expanded across Europe. It suggests PIE may come from the Russian steppes, and that their secret weapon was the domestication of horses (due to the prevalence of "horsey" words in PIE). Diamand uses similar logic to trace the expansion of other cultures. For contrast, New Guinea has lots of of languages, as until recently it has been very divided culturally and politically. Many villages were pretty much their own state.