Invasion of Earth!!

It could have been gradual. A plague or global warming or a supervolcano, any thing could have done it..

 

Earth hasn't been abandoned.....The UED is from Earth. Don't you all remember the propaganda segment at the end of the Terran campaign in BW?

The UED was trying to justify the invasion of the Korpula Sector to its citizens by claiming that the Zerg were going to invade Earth next.

At any rate, before the events of Starcraft, I'm sure that either the Protoss or the Zerg could have easily invaded Earth without much trouble at all. After the events of Starcraft though, both races were severely weakened due to the heavy losses during the chaos that followed the death of the overmind, and I doubt either of them are currently a match for the forces of Earth.

 

Armadeo, prior to the war in starcraft the UED still outnumbered them by millions.

Earth was neither abandoned nor did it succumb to any sort of disaster. Now from what i've read they rounded up about 800,000 thou prisoners and sent them on their way. (the terrans of Koprolu sector) I also remember seeing that the prisoners represented 1 millionth of the Earth's and it's outlying colonies population. That means the UED consists of 800 billion people. I'm pretty sure the Protoss and Zerg cant match that even if they both had mad inter-racial sex.