
Originally Posted by
Visions of Khas
Project Blackstone states that Overlords were assimilated for their psychic sensitivity, and that the Overmind refused to heavily alter them for fear of losing those genetic components that make them so valuable. So its possible that, though capable of space travel, the Overlord's abilities couldn't be passed on to the remainder of the Swarm, necessitating assimilation of the Behemoths.
It's interesting that a species that willingly gave themselves to the Swarm wound up being preserved almost 100%.