Afaik, the thing is, there's no HK-47 out there the team could use. The only models that exist are ported and it can cause potential legal trouble. There was a similar issue with Revan until someone made a model for him.
^ The concern was always "it's a ported model, we could get in trouble for this", and no one made an HK-47 model that was good or optimized well enough for the game.
