The elites died on Hydaelyn
The creatures were practically fantasy-level monsters that the average adventurer could kill...but these invaders came unprepared. The Sleeper Ship circled back around and came back to Earth
Over the millenia, life re-evolved from what organisms survived the comet. The Brontorocs die of indigestion because of being unable to digest carbon-based long pork
Let's be honest. Eating an animal from a completely different planet would screw up any alien digestive sistem.