Earlier quoted context omitted.
Super spoiler below: Their tri-solar system was too unstable to terraform, they needed a stable solar system to migrate to. Of course ours was the nearest with an habitable planet (otherwise there wouldn't be much of a story), so they can immediately colonize Earth, and probably begin the centuries-long process of terraforming the neighboring planets.
Still doesn't make sense. Why limit our tech when they ultimately want to just eliminate us all together? Drop a super virus on us or irradiate the whole planet. Any species capable of disrupting our particle accelerators is more than capable of wiping us from existence.
If you need to wipe out the inhabitants but leave everything else so you can now use it, you need to not destroy everything in the first place. Otherwise, you now have to terraform a planet that you chose because you didn't need to terraform it.