Because he is soka gakkai internationaol's president.

The number of people who practice Nichiren's Buddhism depends upon whether you're counting the Soka Gakkai/Soka Gakkai International or Nichiren Shoshu. within the SGI is in 192+ countries/territories around the world I'm unsure of the exact number of members. I have no idea about Shoshu as I am not affilliated with them.

Vicki Rea Holtzapple has written:

'Soka Gakkai in midwestern America'

Through the practice of Nichiren Daishoni's Buddhism people can positively chage themselves and effect change in their environment. Once one's life is positively changed through this practice, the result is true happiness of that individual. Ultimately, he(or she) will want to share this with others. The Soka Gakkai is an organization that encourages it's members in this Buddhist practice. It continues to grow in membership and currently is present in 192 countries with approx 12 millions members worldwide. The Soka Gakkai contribution to cultural exchange, global peacebuilding (understanding against nuclear and conventional warfare) and the development of youth make it one of the world largest peace orgainzations

Soka Gakkai is particularly concerned with the Lotus Sutra. Anyone who follows the teachings of Nichiren is likely to have said it out loud at some point!