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.
....A quick look on Wookiepedia shows that the first named appearance of a Cathar was in 1994 in a Dark Horse story. That's a good...9 years before Bioware even touched the Star Wars EU. So I'm fairly certain they did not invent the Cathar unless time travel was involved.