Question: Why did Nichiren say, "Shakyamuni Buddha" who resides in our mind and not "Amida, Buddha", "Dainichi Buddha", "Yakuchi Buddha", "I Nichiren", "Joe the Plumber" "Daisaku Ikeda", nor "Buddha-nature"?
Answer: According to Nichiren, some of us received the seed of Buddhahood (Namu Myoho renge kyo) from Shakyamuni Buddha in the remote past and others for the first time in this very life. Either way, we are emanations of Shakyamuni Buddha and this Buddha alone.
Buddha-nature is the field of Buddhahood. We are all fertile fields but without the seed of Buddhahood, the field of our Buddha-nature lay barren. Without the continuous water of the Law (practice and faith in the Gohonzon) the seed lays dormant. Shakyamuni Buddha states,
"My Law is subtle and inscrutable." - Lotus Sutra Chapter 2
Nichiren Daishonin states:
"In fact Sakyamuni began to preach, planting the seed of Buddhahood in the eternal past... Sakyamuni Buddha continued to guide His disciples until they were all sure to attain Buddhahood during the preaching of the Lotus Sutra in His present life, completing the series of His preaching which began in the eternal past." -- Kanjin Honzon-Sho (NOPPA) page 56:
"Demonstrating ten great mystic powers the Buddha (Shakyamuni) transferred Namu Myoho renge Kyo to the four great bodhisattvas" (Ibid Kanjin Honzon Sho)
There are dozens more such passages. SGI confuses Buddha-nature [which even men of the Two Vehicles and those of incorrigible disbelief possess] with Buddhahood. It is thanks to Shakyamuni Buddha that we can obtain Buddhahood. Nichiren's is not a new age philosophy, "we are Buddha's as we are." It is thanks to Shakyamuni Buddha that we can, through a correct faith and practice, develop the seed of Buddhahood into the great tree of Buddhahood. To deny the centrality and ever present reality of Shakyamuni Buddha is to be an unfilial child.