making up their own planets requires less clearance and hassle from LA than getting the ok to use established stuff. Its likely the same reason that Cathar (a race bioware invented) are the first new species as well. This happened all the time in SWG to SOE... useing established names places and characters, no matter how mundane for new content requires multiple hoops, premissions and approvals from lucasarts..so typically its much easier to just make up their own.
I suggest you read the Tales of the Jedi series from Dark Horse comics from the mid-90s to see the Cathar for the first time in the Star Wars universe. The Exar Kun War, for example.