if you consider the huge amount of custom makers in u.s. then certainly we are champs . however there are japanese & russian & german individuals that take a back seat to no one.really esav probably says it best.
there are plenty of excellent custom makers the world over. (eg dave budd and bernie garland, both UK makers)
I think the US's prominence in the field comes from a combination of your greater population, economic development, wideness of available markets, and excellent communications.
greater population leads to a greater pool of people to become knife makers, and a greater potentiall buying public to support them.
economic development has the effect that more people can afford high-end custom knives, allowing custom makers to make a living, or be able to afford the hobby of knife making.
the wideness of the markets is best exemplified with the "tactical knife". compared to the UK and other european countries, the US has a far more developed market for millitary, law enforcement and self-defence equipment, allowing for this new style to develop.
finally communications cover both the internet ect, allowing makers to advertise, and with the larger knife buying public of the US, for word to spread. it also covers the internal communications whithin the country, allowing anyone within the country to order the makers work, where makers in less developed countries might only be able to serve a fairly local area, with any further trade quickly loosing their attached name.