David, a mercenary faction is in it's essence a private military company, so, being a company, earning funds is the ultimate goal. Currently, the Immortals are the wealthiest empire in the galaxy and are constantly hiring mercs for their need of victory. Plus, they give their "allies" a great deal of cutting-edge equipment, boosting your effectiveness on the field. My faction, the MAOX, is currently aligned to the Immortal Empire and it has proved to be a very profitable initiative.
If they emerge victorious from this war, it is in every mercenary faction best interest to be with them. Otherwise I fear you know what happens to those who aren't.
"Virando costas ao mundo, Orgulhosamente sós, Glória antiga, Volta a nós"