They are... how to say this... not well known? Due to BMS having a repository for all their songs, you probably think that there are 10x more compare to o2jam songs, but in fact, bms actually has 20x more lol no just kidding o2jam has quite a lot of 40+ songs. Rather than well known, most of the maps are not publicly released too, and they only share it to specific groups of people.
Most of the time, you'd need to have connections in order to get some maps x_x
I guess it's pretty much like playing some unuploaded maps in osu (all game modes). You can only get it from the creator itself most of the time.