Welcome to fsi-language-courses.com, the home for language courses developed by the Foreign Service Institute. These courses were developed by the United States government and are in the public domain.
This site is dedicated to making these language courses freely available in an electronic format. This site is not affiliated in any way with any government entity; it is an independent, non-profit effort to foster the learning of worldwide languages. Courses here are made available through the private efforts of individuals who are donating their time and resources to provide quality materials for language learning.
I am pretty sure that Foreign Service Institute has Persian Farsi and many other language courses not listed, for what ever reason the volunteers at fsi-language-courses.com (note, two different organizations) had access to Igbo and copied it to electronic format, they may not have had the opportunity to do so with Japanese.
Kind of like Project Gutenberg, the information is not driven by popularity as much as by what is available and what interests the volunteers.