While at first, it seems that the Cabal are here to conquer, to expand their Empire. However, they actually appear to be running from something, most likely Oryx and the Hive. This explains why they stay at the fringes of he inner system: they are afraid of encroaching on Hive territory. The Awoken don't seem to have too much of a beef with them right now. They could possibly create a temporary alliance when Oryx comes