<ol>
<li><p>Hell if I know.</p></li>
<li><p>Because our population was so big and they like our blood.</p></li>
<li><p>I'm thinking that the aliens' life and the machines are tied together. Weak alien -- weak machine, so the force fields would falter.</p></li>
<li><p>Pretty sure he was with the military, at least for a while.</p></li>
</ol>