personally, i think it's because in the UK, tastes are more eclectic and less MOR than in the US...hence the success of such diverse acts as say Spice Girls and at the other end of the spectrum, say, Nirvana (obviously just examples)...
also, because the US is so big, the musical tastes vary so wildly from region to region, an example the early punk scene that emerged in NYC area (Ramones, Stooges, Blondie etc) was, although crap compared to UK scene, 10x more likely to be heard in London than in Nashville...