Once upon a time when SWTOR launched Ilum was promoted as the PvP planet. We had large battles there with hundreds of players but....the engine this game is based on is so crappy it can not handle so many players fighting one another. The result was terrible performance for everyone on Ilum; freezes, crashes etc.. so rather than fixing the engine (lots of work) they just more or less removed large scale open world pvp (far less work).
TLDR: will never happen