They came to find out about us and determine our worth. I mean, yeah, they're judgemental--but really, when you're going around blowing up planets to absorb their mineral content, being selective is a positive thing. That means, at the very least, some worlds will be spared.
So they made the trip, got into a low orbit, and checked us out. Sure, we hate and fight and torture and ignore each other's suffering (usually at our very own peril); but we also give to one another, we can be funny, and there is no doubt that Earth has created some of the finest desserts in the galaxy. In fact, after sampling the Reese's Pieces Sundae at a Friendly's in Harrisburg, Pennsylvania, Deputy Assessment Agent Glorgak recommended sparing our little planet.
They had gathered their various Agents and we're just beginning to break orbit when Chief Arbiter Pleghull was lazily regarding one of the planet's broadcasts. As said planet got smaller and smaller outside the window of his quarters, Pleghull saw that during that very week, the filmed entertainment "Wild Hogs" had broken a number of attendance records and that "The Next Pussycat Doll," a free television broadcast had enjoyed a huge audience as well. Calling those entertainments to his monitor station, Pleghull quickly put in a call to the officer at the ship's helm.
"Reverse course!" he barked.
Pleghull sat, watching both works simultaneously, each with a different eye. It took him roughly twelve kecktos (three minutes) to make his decision.
And that is why our planet was destroyed, Billy. It could've... Well, it should've been different.