Are we certain about this? I don't have a historical dictionary of Japanese handy where I could check. However, I did find 俄羅斯 in an online Hanca dictionary (http://handic.empas.com/), so it apparently had some currency in earlier Korean. For instance, the Russian embassy in Seoul was known during the Choseon era as the 俄館. Perhaps the Japanese version replaced the earlier Chinese one in much the same way that 러시아 (apparently based on the English pronunciation) replaced it?