Here's why:

* Earth's mantle is primarily composed of silicate minerals. These minerals are rich in elements like silicon (Si), oxygen (O), magnesium (Mg), iron (Fe), calcium (Ca), and aluminum (Al).

* Hydrogen is a very light element. It is primarily found in the Earth's atmosphere and oceans. While trace amounts of hydrogen might be present in the mantle, they are negligible compared to the other major elements.

Let me know if you have other questions about Earth's composition!