Possibly, O'Sensei did not speak of the internal because of the inherent attitude of Japanese.
A perspective from writings on Buhda, given to the assembly of monks at Kannondōri in Kōshōhōrin-ji Temple on the fifth day of the summer training period in the second year of the En'o era (May 12, 1240).
page 72 or page 8 in pdf
'Further, this land of Japan is a distant place separated from others by vast seas. The hearts of its people are befuddled in the extreme. From the distant past, it has not been common for saintly persons or those naturally gifted with good sense to be born here, to say nothing of the scarcity of true students of the Way. When the fellow who knows nothing of the heart that seeks the Way is told of this Way-seeking heart, he turns a deaf ear to this good instruction. As a result, he does not reflect upon himself and harbors resentment towards others.
In short, when you put into practice your intention to seek enlightenment, you should not concern yourself with letting worldly people know that you have given rise to the enlightenment-seeking mind and are practicing the Way. Rather, you should conduct yourself so that they may not know it; even more, you should not speak of it openly. People today who seek Truth are rare; as a result, the majority do not engage themselves in spiritual practice and have not awakened in their hearts. Desirous of praise from others, they seek for someone who will tell them how integrated their practice and understanding are. This is just what ‘being deluded within delusion' means. You should immediately toss out such ridiculous notions'