Many nations have a much smaller animation output than the United States and Japan, so it is merely the law of averages that there is less of it being exported from their countries of origin to the United States. Many nations' animation is culturally unacceptable in other part of the world (you really don't want to see what they are calling "children's entertainment" in the Muslim world).
Also, in many cases, other nations have lacked a great visionary on the scale of a Disney or a Tezuka to define an entire genre, thus their cartoons are very derivative of American and Japanese fare. Nor has there been any attempt on the part of American fans to create the kind of cross-cultural appreciation that has been directed toward Japanese animation.
Finally, many foreign cartoon have their overseas origins obscured when they are imported to the States, just as Japanese cartoons were during the 1960's. W.I.T.C.H., Winx Club, Totally Spies, Martin Mystery, and Code Lyoko all originated in Europe.