There are some American Koi Breeders who are doing very good work and developing their own distinct bloodlines, but it is still true that the birthplace of Nishikigoi is Japan, and they have many generations of experience to do it well.
That does not mean that American bred Koi are not excellent because many are, but in Japan Koi is a part of the culture and there are far more breeders there with a strong background than here.