Why did the Vuduri erase our cities?

When Rei first arrived at Skyler Base, he was sorting through the image archives and noticed there was no evidence that our culture ever existed. Not New York, Chicago, Los Angeles, Paris, London - all were gone. During their first trip back to Earth in Part 3 of Rome's Revolution, Rei asked, “Is there anything left of our cities, you know, from my time?” “No,” replied Rome. “I am not sure of the proper word. They were…erased.”

Later, in The Ark Lords Rome did choose to expound upon this a bit further. Rome said, “You will recall you once asked me what happened to all your cities and I told you they were erased. So, too, it was with the Erklirte’s settlement. What I did not tell you was that this was Hanry Ta Jihn’s dying wish. He was a martyr but he was so revered that every person on Earth tried to honor that wish. It took centuries. It wasn’t until the Vuduri came into being that they finally accomplished his goal. Every trace of the Essessoni civilization was razed, burned or buried. In the end, it was to be as if your people never existed.”

Finally, in The Ark Lords we were provided with a little glimpse into the past. We saw the death of Jack Henry (Hanry Ta Jihn in Vuduri) and in this little vignette, the Battle of Chicago is now over and Jack Henry lie dying on ground. Red June, Jack's most trusted lieutenant, had Jack's head resting on his thigh:

    “Did we get ‘em?” Jack whispered.
    “Yeah, Jack,” Red replied. “They’re all dead.”
    Jack grunted. “You have to…” His voice was so quiet, Red couldn’t hear him. He bent over so his ear was next to Jack’s mouth.
    “What is it, Jack?” Jack Henry spoke but his breath barely stirred the air. Red had to strain to hear his words.
    “Use their cannon and level this place. Then go back home and get the rest of them. No matter how many men it takes. Bury them and their vehicles and their spaceship and their weapons. Bury every place they’ve ever been on the Earth. They may have other disease weapons stashed somewhere else.”
    Jack coughed and blood spurted out of his mouth but he went on. “Someday, there may be others, more spacemen. Make it so they can’t find a trace, not a hint, that their race was ever here. Swear to me.”
    “I vow it, Jack,” Red said. A tear formed in the corner of his eye. “Even if it takes us a thousand years, we’ll erase everything. It’ll be like they never existed.”

Jack Henry was satisfied and then he died. And that is why all our cities were erased.