At a minimum, the reasons for the horror would be Lovecraftian or “cosmicist.” The scale of natural reality reduces us to insignificance, which is evidently terrifying because instead of grappling with the harsh existential truth we often personify the First Cause so we can socialize with it and feel at home in the universe. Fear seems a clear motive not just for theistic religion but for our secular zeal in replacing the wilderness with our human-centered artificial environments.
The connection between strangeness and fear may not be logically necessary, since explorers may enjoy discovering the unknown. But there’s a psychological reason why we tend to fear what’s strange: we prefer to socialize, since that’s what makes us happy, and we can’t socialize with a world of impersonal processes and objects. If we can’t socially interact with the foundations of the real world, our social interactions with each other might be threatened since they might seem superficial or a trivial biproduct of an underlying, indifferent order. In response to that threat, we tend to demonize that which is inhuman, to protect our primary resource for making ourselves happy.