You could argue the US has been overrepresented compared to historical precedent for the last couple of generations; the Lemond/Hampsten generation begot a lot of people interested in the sport that otherwise would not have been, and certainly the Lance days will have inspired a lot of people to take up the sport, leading to a period where not just the people who made it alongside him but generations who went pro after taking the sport more seriously following on from those successes led to an increase in the US presence in the péloton. This also begot more US teams at the pro level, with the advent of the WT guaranteeing a spot in the biggest European races for teams, as well as lots of other teams seeing the US as a source of developing riders more than they had done in the past. In recent years, the folding of HTC and the morphing of Radioshack back towards a European team in Trek as it disassociates itself from its origins as an Armstrong vanity project as well as progressive mergers of Garmin with European teams has perhaps lessened the US strength at the WT level, as well as the old problem that, like a lot of countries, the US is a nation where the general knowledge of the cycling calendar is fairly limited, so the Tour has the hugely exaggerated importance, exacerbated massively in the Lance years of course by his selective calendar. And there's no US candidate to win a GT right now; Tejay and Talansky are perhaps the closest but they need a LOT of luck to get a GT they can win at time of writing. The other problem is with fewer US teams at the top level and with other scenes usurping them as a source of young riders to other teams, as well as some non-US sponsors with significant US interest (such as HTC or Rabobank) leaving the sport, there's less cause for so many teams to take the jetlag of travelling over to America for flyaway races in the middle of preparation periods.
And to add to this, the US domestic scene is very different in character from a lot of European races - the style of roads is different, the tendency towards crits and TTs is of a wildly different flavour to the similar races in Europe (compare a classic American four-corner pure speed crit against a Dutch 800-pieces-of-road-furniture crit or a classic Belgian kermesse on narrow concreted roads) and roads on climbs were often made with much more modern equipment which favour a completely different kind of climber. As a result, unless a team has significant US interest and wants to do that full calendar, there may be some scepticism from teams as to whether somebody who goes well in the US domestic scene will adapt to Europe, so may well favour a European rider of comparable level in a direct choice between the two. If you look at the women's péloton, the American women tend to have their success at a later age than many others, as travelling across the Atlantic to base themselves out of Europe is a risk they're often only willing to take once they've finished their studies or have established themselves satisfactorily to get a reasonable salary out of the relocation. Megan Guarnier is two years older than Marta Bastianelli, for example, but Bastianelli's seen as a veteran who's reinvented herself a number of times over a storied career, Megan has really come into her own over the last three seasons.