Here's why:

* Mantle Composition: The Earth's mantle is primarily composed of silicate minerals, with the most abundant elements being oxygen, silicon, magnesium, iron, calcium, and aluminum.

* Hydrogen's Role: Hydrogen is primarily found in the Earth's atmosphere and oceans, and it plays a key role in the formation of water. While some hydrogen is present in the mantle, it's not found in significant quantities compared to other elements.