Nichiren explains why, Namu Myoho renge kyo

“All the doctrines possessed by the Thus Come One, all the freely exercised supernatural powers of the Thus Come One, the storehouse of all the secret essentials of the Thus Come One, all the most profound matters of the Thus Come One— all these are proclaimed, revealed, and clearly expounded in this sutra [Myoho renge kyo].” --Nichiren citing Lotus Sutra Chapter 21