Here's why:

* Biology is the study of life, and bacteria are living organisms. They have their own cellular structures, metabolism, and can reproduce.

* Earth science focuses on the physical aspects of the Earth, including geology, meteorology, oceanography, and astronomy. While bacteria play a role in Earth's ecosystems, their study falls under the realm of biology.

However, it's worth noting that there is some overlap. For example, the study of how bacteria interact with rocks and minerals (biogeochemistry) might fall under both earth science and biology.