If you were an alien, why would you not contact humanity?
Revamp of my previous answer :D
I'm sitting in orbit of Earth, We have hijacked and downloaded everything on the internet, fished out the virus's, and we are know going through Human History.
From their time in Africa, to expanding across the globe. Seeing there nations build from small nation, to magnificent kingdoms and empires, the dark time the black Death, Colonisation,etc.
You ee for mot of history they fought first, asked questions later. You know, as your sitting there looking at there history they are preparing to fight you out.
You also look at their diseases, you know there immune to them but still many carry them. Crap, we're not immune. If we aren't affected by these disease, why would we want this land anyway? I mean, we could just destroy from above. But lets just say we're a peaceful republic in space :D
After reviewing the human history, we determine it just ain't worth it. They've used up most of the useful resources, their diseases would probrly mutate to effect us, hell we might bring a disease and whip out all of humanity, and we don't want that we're a peaceful Republic.
We send one single message so Earth doesn't plummit into chaos or some crap, or fire at us.
"We have determined you are not worth trying, we do not want to spread our diseases to humanity. Although one tip: Stop using you resources, eplace them with more energy sufficent resources. We've seen this before, you will be plummeted into the dark age again."
We speed away. Never being seen again.