Here's why:

* The asthenosphere is a soft, partially molten layer of the Earth's mantle. It's not rigid like the lithosphere (the Earth's crust and uppermost mantle) where earthquakes originate.

* Earthquakes are caused by the sudden release of built-up stress in the Earth's lithosphere. This stress arises from the movement of tectonic plates. The plates are rigid and interact with each other at plate boundaries, causing friction and stress.

* The asthenosphere allows the plates to move. It's like a layer of "lubricant" that allows the tectonic plates to slide over it.

In summary: Earthquakes happen in the rigid lithosphere, not the soft, partially molten asthenosphere.