Lucas decided not to include it in ROTS, in order to focus more on Anakin's story, and had it explained in Labyrinth of Evil. Besides, it was best, IMHO, to leave it the mystery it is in the films. You know someone named Sifoo-Dias ordered the clones, but you know little about his him. So, you are left guessing if it was really Sifo-Dias, whether Dooku/Palpatine forced Sifo-Dias to do it before they killed him, etc.