Well, it has historic reasons - KDE was founded in Germany and many of the initial wave of developers were based there, and it's the same for Gnome and the US. Both were certainly international from the start and the bias on the developer side largely no longer persists (also due to the huge presence of South America and India in FOSS now), but for the first half-decade you were simply far more likely to run into KDE people at a European event and into Gnome people at a US-based event, and thus make connections and become involved. This then extended to things like the prominent distributions (SuSE and Mandrake: KDE-centric and from Germany and France; Red Hat: Gnome-based and from the US) and print mag coverage, when that still mattered. And of course Trolltech was a Norwegian company later bought by a certain Finnish phone maker.
And to become more markedly opinionated for a moment, it's also most likely why Gnome managed to catch on and get to a place where it could make some meaningful contributions to the space, despite always having run on arguably inferior technology: The US FOSS scene had a little bit more of an entrepreneurial spirit going (also due to the early decision for LGPL licensing - well, kdelibs was always LGPL as well, but Qt only went LGPL a few years ago, while GTK+ always was) and the US IT market is simply somewhat bigger as well. If KDE had put some more effort into growing US mindshare early on (like it has tried to in recent years by launching a second annual conference just for the US, Camp KDE) things might have gone differently.