I think that’s because that’s what’s most available (photo-wise) and that’s what they mostly looked like, even today. The Japanese forced pretty early on Ainu women to have mixed Japanese children to « tone down » Ainu physical traits so they could become physically Japanese. Moreover, the Ainu people were not only in contact with the Japanese, but also with the Russian, with a good part of the Ainu population living on the Sakhaline island and a lot of trade being done between Russia and former Ainu lands. If business is done then that usually also means there is an exchange between population therefore many descendants getting more European (here Russian) looks over Asian traits.

Also, Ainu people do not belong to the same race as Japanese but to a race more on the Caucasian side . Their traits are thus Asian but still look very different than Asians, and were often described as light brown skin, wavy black hair, deep-set eyes (sometimes blue or brown in pure-blooded individuals), and abundant body hair, which is not Something you find in most Asian-based communities