My best guess: Dr Zaius is all about fear. He himself feared what would happen if Man ever got the chance to return to power. He wanted the rest of his people to remain in ignorance about how they were not always the rulers of the planet. He wanted humans to remain in fear of Apes, keeping them in everyones eyes, including their own, reduced to nothing but beasts. Dr Zaius is an intellectual, and fear is in the mind. I'm not familiar with all the various Sinestro Corp members, but I doubt there are a lot of dummies if any. Of all the PotA characters, he is most likely to be granted a Sinestro Corp ring.