Here's why:
* Epi- means "upon," "over," or "above."
* Hypo- means "under," "below," or "deficient."
Let's look at some examples:
* Epidermis: The outer layer of skin.
* Hypodermis: The layer of skin beneath the epidermis.
* Epicenter: The point on the Earth's surface directly above the focus of an earthquake.
* Hypocenter: The point within the Earth where an earthquake originates.
So, while not always complete opposites, "hypo-" often serves as the contrasting prefix to "epi-".