Why do the Japanese say “national language” (kokugo)?

In Language and Society in Japan Nanette Gottlieb argues fairly persuasively that it is mainly used by nationalists for nationalistic reasons, and that more neutral and leftwing people prefer the term “Nihongo”.  As the school syllabus still uses “kokugo” but NHK uses “Nihongo”, I think that one word gives quite an insight into where Japanese society is right now.