I think the reason for this is that Toshio focuses on breeding top quality fish in limited numbers for the discerning (and better heeled) serious hobbyist, rather than on volume. For such limited number of top notch fish, the Japanese market is sufficient to absorb their production. The sheer volume of fish being produced by Sakai of Hiroshima every year cannot possibly be offloaded in Japan alone, so they obviously have to concentrate considerable efforts on exports.
I've also been told that the Japanese market for koi has been shrinking somewhat over the last 10 years. The younger generation has not acquired the passion, apparently. Breeders have been concentrating on building up the hobby in export countries.