V is for Vendetta
The Visitors have come to seek vengeance on humanity for what our long-forgotten Ancient Astronaut
ancestors did to them when they were in a primitive state.
The Visitors are Artificial
The Visitors were genetically engineered by Neglectful Precursors
from Earth life. This explains how the Visitor's biology is compatible with their human 'suits', how they can seemingly digest human food, and how Brian can get Robin pregnant
. They need Earth in particular, rather than the bazillions of other planets in the galaxy, because as their original homeworld it is uniquely suited to their physiology.
The Visitors are actually from Earth in the first place.
This is even foreshadowed in the second part of the first miniseries when a scientist talks about how the dinosaurs would have evolved into intelligent tool users. The Visitors evolved then, saw the asteroid coming and managed to escape the planet. They went to Sirius in a trip that took millions of years via Generation Ships
. By the time they got there, they only had a few thousand years to develop faster-than-light drives. Eventually they wore their new home planet out of resources and turned their eyes back towards their own home. This also explains why the visitors can eat Earth lifeforms.
Oh, the resistance was all about democracy, but them silly humans never learn. With the visitors gone (ignoring the TV series), Earth went totalitarian and paranoid in setting itself up to be defended from any and all outside threats. Radchack is obviously Ham's Identical Grandson