Well you do have to beat them, into submission before they will help you... Besides, we have a 1000 year gap and the Epsers appear on the other side of the continent by this point. I get the feeling that alot happened between XII and Tactics, like the Great Cataclysm mentioned in the XII Ultimania. In fact, it may even have to do with the events of XII. The gods no longer have control over humanity so perhaps the Lucavi decided to take their absence as a new chance to revolt. Its just a theory of course.